80 



TEXT-BOOK OF EMBRYOLOGY. 



appears behind the second, then a fourth pair, and so on through the 



series. 



Developing from before backward, the somites here as in Amphioxus and 

 the frog again illustrate the general principle that growth progresses from 

 before backward. The first somites appear only a short distance in front 

 of the primitive streak, about where the anterior end of the primitive axis 

 was located, and the paraxial band of mesoderm between the somites and the 

 streak is wholly unsegmented. Then, as successive somites appear and the 

 band becomes thus far segmented, new cells are constantly added at the 

 caudal end of the band as the primitive streak recedes, so that eventually 

 the whole series of somites (more than 40 pairs in the chick) is still followed 

 by the primitive streak which now lies at' the caudal end of the embryo. 



Ectoderm 



Neural Primitive 

 tube segment 



Entoderm 



Coelom 



FIG. 52. Transverse section of chick embryo (2 days incubation). Photograph. 

 The parietal mesoderm (lying above the coelom) is not labeled. The two large vessels under 

 the primitive segments are the primitive aortae. Spaces separating germ layers are due to 

 shrinkage. 



Each somite is composed of massive epithelium whose cells converge 

 toward a small central cavity (myoccel). The latter cavity, however, 

 contains, a few loosely arranged cells (Fig. 52). Laterally each somite is 

 continuous with a much thinner mass or plate of cells, known as the inter- 

 mediate cell mass or nephrotome, which in turn merges with the lateral 

 mesoderm of the area pellucida. 



The sheet of lateral mesoderm becomes separated into two plates or 

 layers, an outer which is apposed to ectoderm and an inner apposed to ento- 

 derm. In many places small clefts appear among the cells, grow larger and 

 finally coalesce to form a continuous cavity, the ccelom, which in its greatest 



