80 



THE EARLY DEVELOPMENT OF MAMMALS. 



and were, therefore, called the proto-vertebrce. This name is still used, although 

 the idea, upon which it was based, is known to be erroneous because the primi- 

 tive segments form much more than the vertebrae. 



The association in time of the development of the medullary groove and 

 primitive segments is important. By the formation of the groove the space 

 between the ectoderm and entoderm alongside the groove is increased, and it is 

 this space which gives the mesoderm the opportunity to grow in thickness so as 

 to form the segmental zone next to the medullary groove. 



In the amniota when the primitive segments are first formed they contain 

 no actual cavity, but we must consider that one is morphologically present, since 

 we can easily observe the line of contact between the opposite walls of the seg- 

 ments. As observed in transverse sections the segments when first developed 

 are triangular in outline. The base of the triangle extends along the side of the 



7^^ Spl 



Fig. 32. — Transverse Section from a Chick Embryo with about Eighteen Segments. 

 Only the mesoderm of one side has been drawn. The section passes through a recently formed segment. My, 

 Secondary segment. C, Core of the segment. W.d, Wolffian duct. N, Nephrotome. Cae, Coelom. 

 Sonty Somatic mesoderm. Spl, Splanchnic mesoderm. X 227 diams. 



medullary canal; the apex of the triangle lies next to the splanchnocele, and at 

 the point of the triangle the somatic and splanchnic mesoderm separate widely 

 from one another. Very soon the apex of the triangle forms a narrower piece 

 (Fig. 32, A^), which is known commonly as the nephrotome or intermediate cell 

 mass. While the nephrotome is being marked off the proximal portion of the 

 segment enlarges, the cells assume a more distinctly epithelial character (Fig. 

 32, My), enclosing a considerable space, which, however, is completely filled by 

 a mass of cells, C, which arise by a proliferation of the cells from the lower side of 

 the segment. The line around this mass of cells marking it off from the other 

 wall of the segment indicates the morphological cavity. In the sheep and the 

 chick it has been observed that the cavities of the first four segments can be 

 traced through the nephrotome to the splanchnocele. This represents a primi- 

 tive condition, one which we find in all the segments of some of the lower verte- 



