The Evolution of Man, vol 2, Ernst Haeckel [online e reader txt] 📗

In the human embryo and that of all the other Amniotes the lungs develop from the hind part of the ventral wall of the head-gut (Figure 1.149). Immediately behind the single structure of the thyroid gland a median groove, the rudiment of the trachea, is detached from the gullet. From its hinder end a couple of vesicles develop—the simple tubular rudiments of the right and left lungs. They afterwards increase considerably in size, fill the greater part of the thoracic cavity, and take the heart between them. Even in the frogs we find that the simple sac has developed into a spongy body of peculiar froth-like tissue. The originally short connection of the pulmonary sacs with the head-gut extends into a long, thin tube. This is the wind-pipe (trachea); it opens into the gullet above, and divides below into two branches which go to the two lungs. In the wall of the trachea circular cartilages develop, and these keep it open. At its upper end, underneath its pharyngeal opening, the larynx is formed—the organ of voice and speech. The larynx is found at various stages of development in the Amphibia, and comparative anatomists are in a position to trace the progressive growth of this important organ from the rudimentary structure of the lower Amphibia up to the elaborate and delicate vocal apparatus that we have in the larynx of man and of the birds.

We must refer here to an interesting rudimentary organ of the respiratory gut, the thyroid gland, the large gland in front of the larynx, that lies below the “Adam’s apple,” and is often especially developed in the male sex. It has a certain function—not yet fully understood—in the nutrition of the body, and arises in the embryo by constriction from the lower wall of the pharynx. In many mining districts the thyroid gland is peculiarly liable to morbid enlargement, and then forms goitre, a growth that hangs at the front of the neck. But it is much more interesting phylogenetically. As Wilhelm Muller, of Jena, has shown, this rudimentary organ is the last relic of the hypobranchial groove, which we considered in a previous chapter, and which runs in the middle line of the gill-crate in the Ascidia and Amphioxus, and conveys food to the stomach. (Cf. Chapter 2.16, Figure 2.246). We still find it in its original character in the larvae of the Cyclostomes (Figures 2.355 and 2.356).

The second section of the alimentary canal, the trunk or hepatic gut, undergoes not less important modifications among our vertebrate ancestors than the first section. In tracing the further development of this digestive part of the gut, we find that most complex and elaborate organs originate from a very rudimentary original structure. For clearness we may divide the digestive gut into three sections: the fore gut (with oesophagus and stomach), the middle gut (duodenum, with liver, pancreas, jejunum, and ileum, and the hind gut (colon and rectum). Here again we find vesicular growths or appendages of the originally simple gut developing into a variety of organs. Two of these embryonic structures, the yelk-sac and allantois, are already known to us. The two large glands that open into the duodenum, the liver and pancreas, are growths from the middle and most important part of the trunk-gut.

Immediately behind the vesicular rudiments of the lungs comes the section of the alimentary canal that forms the stomach (Figures 2.353 d and 2.354 b). This sac-shaped organ, which is chiefly responsible for the solution and digestion of the food, has not in the lower Vertebrates the great physiological importance and the complex character that it has in the higher. In the Acrania and Cyclostomes and the earlier fishes we can scarcely distinguish a real stomach; it is represented merely by the short piece from the branchial to the hepatic gut. In some of the other fishes also the stomach is only a very simple spindle-shaped enlargement at the beginning of the digestive section of the gut, running straight from front to back in the median plane of the body, underneath the vertebral column. In the mammals its first structure is just as rudimentary as it is permanently in the preceding. But its various parts soon begin to develop. As the left side of the spindle-shaped sac grows much more quickly than the right, and as it turns considerably on its axis at the same time, it soon comes to lie obliquely. The upper end is more to the left, and the lower end more to the right. The foremost end draws up into the longer and narrower canal of the oesophagus. Underneath this on the left the blind sac (fundus) of the stomach bulges out, and thus the later form gradually develops (Figures 2.349 and 1.184 e). The original longitudinal axis becomes oblique, sinking below to the left and rising to the right, and approaches nearer and nearer to a transverse position. In the outer layer of the stomach-wall the powerful muscles that accomplish the digestive movements develop from the gut-fibre layer. In the inner layer a number of small glandular tubes are formed from the gut-gland layer; these are the peptic glands that secrete the gastric juice. At the lower end of the gastric sac is developed the valve that separates it from the duodenum (the pylorus, Figure 2.349 d).

Underneath the stomach there now develops the disproportionately long stretch of the small intestine. The development of this section is very simple, and consists essentially in an extremely rapid and considerable growth lengthways. It is at first very short, quite straight, and simple. But immediately behind the stomach we find at an early stage a horseshoe-shaped bend and loop of the gut, in connection with the severance of the alimentary canal from the yelk-sac and the development of the first mesentery. The thin delicate membrane that fastens this loop to the ventral side of the vertebral column, and fills the inner bend of the horseshoe formation, is the first rudiment of the mesentery (Figure 1.147 g). We find at an early stage a considerable growth of the small intestine; it is thus forced to coil itself in a number of loops. The various sections that we have to distinguish in it are differentiated in a very simple way—the duodenum (next to the stomach), the succeeding long jejunum, and the last section of the small intestine, the ileum.

From the duodenum are developed the two large glands that we have already mentioned—the liver and pancreas. The liver appears first in the shape of two small sacs, that are found to the right and left immediately behind the stomach (Figures 2.353 f, and 2.354 c). In many of the lower Vertebrates they remain separate for a long time (in the Myxinoides throughout life), or are only imperfectly joined. In the higher Vertebrates they soon blend more or less completely to form a single large organ. The growth of the liver is very brisk at first. In the human embryo it grows so much in the second month of development that in the third it occupies by far the greater part of the body-cavity (Figure 2.357). At first the two halves develop equally; afterwards the left falls far behind the right. In consequence of the unsymmetrical development and turning of the stomach and other abdominal viscera, the whole liver is now pushed to the right side. Although the liver does not afterwards grow so disproportionately, it is comparatively larger in the embryo at the end of pregnancy than in the adult. Its weight relatively to that of the whole body is 1 : 36 in the adult, and 1 : 18 in the embryo. Hence it is very important physiologically during embryonic life; it is chiefly concerned in the formation of blood, not so much in the secretion of bile.

Immediately behind the liver a second large visceral gland develops from the duodenum, the pancreas or sweetbread. It is wanting in most of the lowest classes of Vertebrates, and is first found in the fishes. This organ is also an outgrowth from the gut.

The last section of the alimentary canal, the large intestine, is at first in the embryo a very simple, short, and straight tube, which opens behind by the anus. It remains thus throughout life in the lower Vertebrates. But it grows considerably in the mammals, coils into various folds, and divides into two sections, the first and longer of which is the colon, and the second the rectum. At the beginning of the colon there is a valve (valvula Bauhini) that separates it from the small intestine. Immediately behind this there is a sac-like growth, which enlarges into the caecum (Figure 2.357 v). In the plant-eating mammals this is very large, but it is very small or completely atrophied in the flesh-eaters. In man, and most of the apes, only the first portion of the caecum is wide; the blind end-part of it is very narrow, and seems later to be merely a useless appendage of the former. This “vermiform appendage” is very interesting as a rudimentary organ. The only significance of it in man is that not infrequently a cherry-stone or some other hard and indigestible matter penetrates into its narrow cavity, and by setting up inflammation and suppuration causes the death of otherwise sound men. Teleology has great difficulty in giving a rational explanation of, and attributing to a beneficent Providence, this dreaded appendicitis. In our plant-eating ancestors this rudimentary organ was much larger and had a useful function.

Finally, we have important appendages of the alimentary tube in the bladder and urethra, which belong to the alimentary system. These urinary organs, acting as reservoir and duct for the urine excreted by the kidneys, originate from the innermost part of the allantoic pedicle. In the Dipneusts and Amphibia, in which the allantoic sac first makes its appearance, it remains within the body-cavity, and functions entirely as bladder. But in all the Amniotes it grows far outside of the body-cavity of the embryo, and forms the large embryonic “primitive bladder,” from which the placenta develops in the higher mammals. This is lost at birth. But the long stalk or pedicle of the allantois remains, and forms with its upper part the middle vesico-umbilical ligament, a rudimentary organ that goes in the shape of a solid string from the vertex of the bladder to the navel. The lowest part of the allantoic pedicle (or the “urachus”) remains hollow, and forms the bladder. At first this opens into the last section of the gut in man as in the lower Vertebrates; thus there is a real cloaca, which takes off both urine and excrements. But among the mammals this cloaca is only permanent in the Monotremes, as it is in all the birds, reptiles, and amphibia. In all the other mammals (marsupials and placentals) a transverse partition is afterwards formed, and this separates the urogenital aperture in front from the anus-opening behind. (Cf. Chapters 2.22 and 2.29.)

CHAPTER 2.28. EVOLUTION OF THE VASCULAR SYSTEM.

The use that we have hitherto made of our biogenetic law will give the reader an idea how far we may trust its guidance in phylogenetic investigation. This differs considerably in the various systems of organs; the reason is that heredity and variability have a very different range in these systems. While some of them faithfully preserve the original palingenetic development inherited from earlier animal ancestors, others show little trace of this rigid heredity; they are rather disposed to follow new and divergent CENOGENETIC lines of development in consequence of adaptation. The organs of the first kind represent the CONSERVATIVE element in the multicellular state of the human frame, while the latter represent the PROGRESSIVE element. The course of historic development is a result of the correlation of the two tendencies, and they must be carefully