84 



TEXT-BOOK OF EMBRYOLOGY. 



As described above, the mesoderm of the chick is at first a solid plate of cells. 

 The cavity in the mesoderm — the ccelom — appears as the result of a splitting 

 of the originally solid mesoderm layer into two sublayers — the parietal and the 

 visceral (Fig. 72). At the same time that portion of the mesoderm which lies 

 adjacent to the neural groove on both sides of the medial line becomes differen- 

 tiated into two series of bilaterally symmetrical segments — the primitive seg- 

 ments, which are connected with one another by intermediate thinner parts 

 (Figs. 73, 74 and 72). The splitting of the mesoderm to form the ccelom begins 

 some distance from the medial line and progresses both laterally and medially. 



Neuropore 



Fore-brain vesicle 



Head fold 



Area pellucida S 



Area vasculosa 

 Area opaca - 

 Yolk - 



Edge of 

 blastoderm 



Mid- and hind- 

 brain vesicles 



Neural fold 



Fig. 74. — Dorsal view of chick embryo with ten pairs of primitive segments. Bonnet. 



The ccelom does not, however, reach the primitive segments, for a small solid 

 mass of cells — the intermediate cell mass (Fig. 81) — always intervenes between 

 the ccelom and the segments. Furthermore, the ccelom from the beginning 

 shows no segmentation. 



The formation of the neural groove and neural tube from the ectoderm and 

 the separation of the chorda anlage from the entoderm are much the same as in 

 the frog. A decided difference is, however, to be noted in the shape of the 

 chick's blastoderm. Since in this case the yolk plays but a small part in seg- 

 mentation, the germ layers at first lie flat upon the surface of the yolk, the 



