Vertebrate Embryology

The great glands of the intestine arise in the embryo as pocket like diverticula of its wall. The pancreas is peculiar in that it arises normally from three distinct diverticula—a pair situated ventrally in the neighbourhood of the bile-duct, and the third dorsal. The pancreas of the adult is formed by the fusion of these three rudiments and it may retain (birds) all three openings to the intestine, i.e., three distinct pancreatic ducts.

Coelomic Organs.

The dorsal portions of the mesoderm seg ments or myotomes form the primitive muscular equipment of the vertebrate. In Lepidosiren both lateral and mesial walls of the myotome undoubtedly become converted into myoblasts or muscle cells, but in most vertebrates the phenomena are obscure and many hold that only the mesial wall gives rise to muscle, the outer breaking up into mesenchyme. In Lepidosiren the early myoblast has the form of a typical myo-epithelial cell continued into the central nervous system by a protoplasmic tail and it is seen that the so-called end-plate of the motor nerve is simply a portion of the muscle fibre which retains its original protoplasmic condition. The Myoto?nes form the foundation of the whole system of voluntary muscles. In fish they remain conspicuous through life.

In elasmobranch embryos it is possible to observe the musculature of the limbs sprouting out from the lower ends of certain myo tomes. In the higher vertebrates the myotomes are conspicuous in the embryo, but in the adult they are no longer recognizable.

The Splanchnocoele or body-cavity becomes normally sub divided into a smaller pericardiac and a larger peritoneal cavity.

In some of the more archaic vertebrates, e.g., elasmobranchs, the two cavities remain continuous through a narrow pericardio peritoneal canal.

The loss of the primitive coelomic segmentation has brought with it characteristic modifications in the development of the archinephros or series of nephridial tubes.

Incidentally it should be noted that confusion is apt to arise owing to the term nephridium having come to be used in two different senses. The term was invented by E. Ray Lankester (1877) as the equivalent of renal or kidney tubule. It is in this original sense that it is used here. Many writers however, follow ing E. S. Goodrich, separate off renal tubules possessing an open funnel (coelomostome) at their inner end under the separate name coelomoduct.

It is a normal characteristic of the embryo of metamerically segmented animals that the head end, with its special nerve centres and sense-organs, develops first, the process of develop ment spreading slowly tailwards, and this principle applies to the aephridial tubes as to other organs. The disappearance of the :oelomic septa in the Vertebrata has however eliminated the neces iity of each segment having its independent pair of drainage tubes. There has accordingly come about a loss of the serial regularity in the development of the nephridial tubes and these tend to develop in three successive batches, known as pronephros, mesonephros and metanephros. Of these the pronephros is the functional kidney in the early stages of those vertebrates in which these stages are free-living (larval). Where, on the other hand, early development is embryonic, it no longer becomes a functional organ, the excretory products presumably passing away by diffu sion from the extensive network of blood-vessels on the surface of the yolk. The pronephros is purely larval: eventually it atrophies, a pair of its nephrostomes however persisting in greatly enlarged form as the ostia or internal openings of the oviducts. The excretory function is now carried out by the series of tubules extending back to the region of the cloaca and termed the opis thonephros. In the fishes and amphibians, this constitutes the kidney of the adult, though already in many of these, e.g., elasmo branchs and urodeles, there is seen a tendency for the excretory function to become concentrated in the hind portion of the opis thonephros, its front portion remaining small and serving for the transmission of the spermatozoa. This condition foreshadows that of the Amniota in which the opisthonephros has become completely divided into mesonephros and metanephros. The former, representing the greater part of the opisthonephros, acts as the functional kidney during embryonic life but later becomes purely reproductive, forming the epididymis attached to the testis. The metanephros—the extreme hind end of the opisthone phros, in which the tubules become greatly increased in size and number—alone forms the kidney of the adult amniote.