972 FURTHER DEVELOPMENT OF THE BODY. 



of the contents of the yolk-sac the vascular area becomes steadily 

 diminished. Finally, toward the end of the period of hatching, the 

 yolk-sac, which has become smaller, slips into the abdominal cavity. 

 Upon the umbilical vesicle of mammals the circulation usually disappears 

 at an early date and the vesicle becomes transformed into a tiny ap- 

 pendage, while the second circulation develops to supplant the omphalo- 

 mesenteric circulation. The first vessels in birds are formed outside 

 the embryonal body in the area vasculosa as early as the last quarter 

 of the first day, even before the heart can be seen. The vessels develop 

 from vasoformative cells of the blood-islands, which at first appear 

 isolated and then become confluent, and whose origin, whether from 

 mesoblast or entoblast, has not yet been determined. At first solid, 

 they later become hollowed out. In mammals (sheep) the first vessels 

 also appear outside the embryo ; the first blood-corpuscles are formed in 

 the region of the vascular area as a product of the endothelium 



Within the area vasculosa of the chick, there develops a closer-meshed 

 lymphatic canal-system, which communicates with the amniotic cavity. 



FURTHER DEVELOPMENT OF THE BODY. 



The formative processes still wanting and necessary for the typical 

 development of the body are as follows: 



1. The ccelom gradually increases in extent, and in consequence 

 the differentiation between the body-wall and the intestinal canal 

 becomes the more distinct. The latter moves away from the primi- 

 tive vertebrae, the middle plate becoming elongated' to form the be- 

 ginning mesentery. The body-wall, which, at first, still consists of 

 the epidermis and the external lamella of the lateral plate (cutaneous 

 plate), undergoes thickening, the primitive muscle growing from the 

 muscle-plate, and the primitive bone, together with the spinal nerves, 

 from the primitive vertebrae beneath the epidermis into the body -wall. 



2. From the primitive vertebrae there is detached a portion situated 

 dorsally, which is designated the muscle-plate. The remaining por- 

 tion of the primitive vertebra (true primitive vertebra) now unites 

 with its fellow of the opposite side, both growing completely around 

 the chorda (membrana reuniens inferior; in dogs on the third, in rabbits 

 on the tenth day), and also enclosing the medullary canal (membrana 

 reuniens superior; in chicks on the fourth day). Thus, there has 

 taken place in front of the medullary canal a union of the primitive 

 vertebral masses that enclose the chorda and therefore form the basis 

 of all of the vertebral bodies, while the membrana reuniens su- 

 perior, interposed between muscle-plates, and epidermis on the one 

 side and the medullary canal on the other side, represents the 

 rudiment of the entire system of vertebral arches, together with the 

 intervertebral ligaments between them. The spinal column is in this 

 membranous stage an exact reproduction of the spinal column of the 

 cyclostomes (lamprey). From the membrana reuniens superior there 

 are formed, besides, the membranes of the spinal cord and the spinal 

 ganglia and nerves. 



TT A~ n rare cases ^e formation of the membrana reuniens superior does not occur. 



Under such circumstances the medullary canal is covered posteriorly by 



the horny layer (epidermis) alone, either throughout its entire extent, or only in 



mted areas 1 his defect in development is known as spina bifida (at the head, 



cephalus) . Failure in the development of the membrana reuniens inferior 



