The Evolution of Man, vol 1, Ernst Haeckel [books to read this summer TXT] 📗

looked for in vain, is found always, and is nothing else than the familiar “primitive groove.” Of this we shall see more as we proceed. Meantime we realise that gastrulation may be reduced to one and the same process in all the vertebrates. Moreover, the various forms it takes in the invertebrates can always be reduced to one of the four types of segmentation described above. In relation to the distinction between total and partial segmentation, the grouping of the various forms is as follows:—

1. Palingenetic (primitive segmentation) 1.1. Equal segmentation (bell-gastrula).

1.1.A. Total segmentation (without independent food-yelk).

2. Cenogenetic segmentation (modified by adaptation).

2.2. Unequal segmentation (hooded gastrula).

2.2.A. Total segmentation (without independent food-yelk).

2.3. Discoid segmentation (discoid gastrula).

2.3.B. Partial segmentation (with independent food-yelk).

2.4. Superficial segmentation (spherical gastrula).

2.4.B. Partial segmentation (with independent food-yelk).

The lowest metazoa we know—namely, the lower zoophyta (sponges, simple polyps, etc.)—remain throughout life at a stage of development which differs little from the gastrula; their whole body consists of two layers of cells. This is a fact of extreme importance. We see that man, and also other vertebrates, pass quickly through a stage of development in which they consist of two layers, just as these lower zoophyta do throughout life. If we apply our biogenetic law to the matter, we at once reach this important conclusion. “Man and all the other animals which pass through the two-layer stage, or gastrula-form, in the course of their embryonic development, must descend from a primitive simple stem-form, the whole body of which consisted throughout life (as is the case with the lower zoophyta to-day) merely of two cell-strata or germinal layers.” We will call this primitive stem-form, with which we shall deal more fully later on, the gastraea—that is to say, “primitive-gut animal.”

According to this gastraea-theory there was originally in all the multicellular animals ONE ORGAN with the same structure and function.

This was the primitive gut; and the two primary germinal layers which form its wall must also be regarded as identical in all. This important homology or identity of the primary germinal layers is proved, on the one hand, from the fact that the gastrula was originally formed in the same way in all cases—namely, by the curving of the blastula; and, on the other hand, by the fact that in every case the same fundamental organs arise from the germinal layers. The outer or animal layer, or ectoderm, always forms the chief organs of animal life—the skin, nervous system, sense-organs, etc.; the inner or vegetal layer, or entoderm, gives rise to the chief organs of vegetative life—the organs of nourishment, digestion, blood-formation, etc.

In the lower zoophyta, whose body remains at the two-layer stage throughout life, the gastraeads, the simplest sponges (Olynthus), and polyps (Hydra), these two groups of functions, animal and vegetative, are strictly divided between the two simple primary layers. Throughout life the outer or animal layer acts simply as a covering for the body, and accomplishes its movement and sensation. The inner or vegetative layer of cells acts throughout life as a gut-lining, or nutritive layer of enteric cells, and often also yields the reproductive cells.

The best known of these “gastraeads,” or “gastrula-like animals,” is the common fresh-water polyp (Hydra). This simplest of all the cnidaria has, it is true, a crown of tentacles round its mouth. Also its outer germinal layer has certain special modifications. But these are secondary additions, and the inner germinal layer is a simple stratum of cells. On the whole, the hydra has preserved to our day by heredity the simple structure of our primitive ancestor, the gastraea (cf. Chapter 2.19.)

In all other animals, particularly the vertebrates, the gastrula is merely a brief transitional stage. Here the two-layer stage of the embryonic development is quickly succeeded by a three-layer, and then a four-layer, stage. With the appearance of the four superimposed germinal layers we reach again a firm and steady standing-ground, from which we may follow the further, and much more difficult and complicated, course of embryonic development.

SUMMARY OF THE CHIEF DIFFERENCES IN THE OVUM-SEGMENTATION AND

GASTRULATION OF ANIMALS.

The animal stems are indicated by the letters a-g: a Zoophyta. b Annelida. c Mollusca.

d Echinoderma. e Articulata. f Tunicata. g Vertebrata.

1. Total Segmentation. Holoblastic ova. Gastrula without separate food-yelk. Hologastrula.

1.1. Primitive Segmentation. Archiblastic ova. Bell-gastrula (archigastrula.)

a. Many lower zoophyta (sponges, hydrapolyps, medusae, simpler corals).

b. Many lower annelids (sagitta, phoronis, many nematoda, etc., terebratula, argiope, pisidium).

c. Some lower molluscs.

d. Many echinoderms.

e. A few lower articulata (some brachiopods, copepods: Tardigrades, pteromalina).

f. Many tunicata.

g. The acrania (amphioxus).

1.2. Unequal Segmentation. Amphiblastic ova. Hooded-gastrula (amphigastrula).

a. Many zoophyta (sponges, medusae, corals, siphonophorae, ctenophora).

b. Most worms.

c. Most molluscs.

d. Many echinoderms (viviparous species and some others).

e. Some of the lower articulata (both crustacea and tracheata).

f. Many tunicata.

g. Cyclostoma, the oldest fishes, amphibia, mammals (not including man).

2. Partial Segmentation. Meroblastic ova. Gastrula with separate food-yelk. Merogastrula.

2.3. Discoid Segmentation. Discoblastic ova. Discoid gastrula.

c. Cephalopods or cuttlefish.

e. Many articulata, wood-lice, scorpions, etc.

g. Primitive fishes, bony fishes, reptiles, birds, monotremes.

2.4. Superficial Segmentation. Periblastic ova. Spherical-gastrula.

e. The great majority of the articulata (crustaceans, myriapods, arachnids, insects).

CHAPTER 1.9. THE GASTRULATION OF THE VERTEBRATE.*

(* Cf. Balfour’s Manual of Comparative Embryology volume 2; Theodore Morgan’s The Development of the Frog’s Egg.) The remarkable processes of gastrulation, ovum-segmentation, and formation of germinal layers present a most conspicuous variety. There is to-day only the lowest of the vertebrates, the amphioxus, that exhibits the original form of those processes, or the palingenetic gastrulation which we have considered in the preceding chapter, and which culminates in the formation of the archigastrula (Figure 1.38).

In all other extant vertebrates these fundamental processes have been more or less modified by adaptation to the conditions of embryonic development (especially by changes in the food-yelk); they exhibit various cenogenetic types of the formation of germinal layers.

However, the different classes vary considerably from each other. In order to grasp the unity that underlies the manifold differences in these phenomena and their historical connection, it is necessary to bear in mind always the unity of the vertebrate-stem. This “phylogenetic unity,” which I developed in my General Morphology in 1866, is now generally admitted. All impartial zoologists agree to-day that all the vertebrates, from the amphioxus and the fishes to the ape and man, descend from a common ancestor, “the primitive vertebrate.”

Hence the embryonic processes, by which each individual vertebrate is developed, must also be capable of being reduced to one common type of embryonic development; and this primitive type is most certainly exhibited to-day by the amphioxus.

It must, therefore, be our next task to make a comparative study of the various forms of vertebrate gastrulation, and trace them backwards to that of the lancelet. Broadly speaking, they fall first into two groups: the older cyclostoma, the earliest fishes, most of the amphibia, and the viviparous mammals, have holoblastic ova—that is to say, ova with total, unequal segmentation; while the younger cyclostoma, most of the fishes, the cephalopods, reptiles, birds, and monotremes, have meroblastic ova, or ova with partial discoid segmentation. A closer study of them shows, however, that these two groups do not present a natural unity, and that the historical relations between their several divisions are very complicated. In order to understand them properly, we must first consider the various modifications of gastrulation in these classes. We may begin with that of the amphibia.

The most suitable and most available objects of study in this class are the eggs of our indigenous amphibia, the tailless frogs and toads, and the tailed salamander. In spring they are to be found in clusters in every pond, and careful examination of the ova with a lens is sufficient to show at least the external features of the segmentation.

In order to understand the whole process rightly and follow the formation of the germinal layers and the gastrula, the ova of the frog and salamander must be carefully hardened; then the thinnest possible sections must be made of the hardened ova with the microtome, and the tinted sections must be very closely compared under a powerful microscope.

The ova of the frog or toad are globular in shape, about the twelfth of an inch in diameter, and are clustered in jelly-like masses, which are lumped together in the case of the frog, but form long strings in the case of the toad. When we examine the opaque, grey, brown, or blackish ova closely, we find that the upper half is darker than the lower. The middle of the upper half is in many species black, while the middle of the lower half is white. ( The colouring of the eggs of the amphibia is caused by the accumulation of dark-colouring matter at the animal pole of the ovum. In consequence of this, the animal cells of the ectoderm are darker than the vegetal cells of the entoderm. We find the reverse of this in the case of most animals, the protoplasm of the entoderm cells being usually darker and coarser-grained.) In this way we get a definite axis of the ovum with two poles. To give a clear idea of the segmentation of this ovum, it is best to compare it with a globe, on the surface of which are marked the various parallels of longitude and latitude. The superficial dividing lines between the different cells, which come from the repeated segmentation of the ovum, look like deep furrows on the surface, and hence the whole process has been given the name of furcation. In reality, however, this “furcation,” which was formerly regarded as a very mysterious process, is nothing but the familiar, repeated cell-segmentation. Hence also the segmentation-cells which result from it are real cells.

(FIGURE 1.40. The cleavage of the frog’s ovum (magnified ten times). A stem-cell. B the first two segmentation-cells. C four cells. D eight cells (4 animal and 4 vegetative). E twelve cells (8 animal and 4

vegetative). F sixteen cells (8 animal and 8 vegetative). G

twenty-four cells (16 animal and 8 vegetative). H thirty-two cells. I forty-eight cells. K sixty-four cells. L ninety-six cells. M 160 cells (128 animal and 32 vegetative).

(FIGURES 1.41 TO 1.44. Four vertical sections of the fertilised ovum of the toad, in four successive stages of development. The letters have the same meaning throughout: F segmentation-cavity. D covering of same (D dorsal half of the embryo, P ventral half). P yelk-stopper (white round field at the lower pole). Z yelk-cells of the entoderm (Remak’s “glandular embryo”). N primitive gut cavity (progaster or Rusconian alimentary cavity). The primitive mouth (prostoma) is closed by the yelk-stopper, P. s partition between the primitive gut cavity (N) and the segmentation cavity (F). k k apostrophe, section of the large circular lip-border of the primitive mouth (the Rusconian anus).

The line of dots between k and k apostrophe indicates the earlier connection of the yelk-stopper (P) with the central mass of the yelk-cells (Z). In Figure 1.44 the ovum has turned 90 degrees, so that the back of the embryo is uppermost and the ventral side down. (From Stricker.)).

The unequal segmentation which we observe in the ovum of the amphibia has the special feature of beginning at the upper and darker pole (the north pole of the terrestrial globe in our illustration), and slowly advancing towards the lower and brighter pole (the south pole). Also the upper and darker hemisphere remains in this position throughout the course of the segmentation, and its cells multiply much more briskly. Hence the cells of the lower hemisphere are found to be larger and less numerous. The cleavage of the stem-cell (Figure 1.40

A) begins with the formation of a complete furrow, which starts from the north pole and reaches to the south (B). An hour later a second furrow arises