30 Vertebrate Embryology 



pleure. The space left between its two layers 

 by this cleavage of the mesoblast will become 

 the body-cavity, or ccelom (Figs. 1 3, C, and 1 5). 

 As the yolk-plug is withdrawn within the egg, 

 the lateral lips of the blastopore come together 

 to form a slight ridge or streak of tissue, the 

 primitive streak, in the centre of which is a 

 narrow chink or groove, the primitive groove 



FIG. ii. STAGES IN THE EARLY DEVELOPMENT OF THE FROG, 



SEEN OBLIQUELY FROM THE POSTERIOR END. (After Zicgler, from 



Marshall.) 



A, yolk-plug stage. 5, primitive streak and early neural folds. C, later 

 neural folds. Z?, closure of neural canal and beginning of tail. E, completion of 

 neural tube, closure of blastopore, presence of proctodaeum, increase in tail. 



(Fig. n, B, C, and Z>). The primitive streak 

 is very inconspicuous in the frog, but, as will 

 be seen later, in the chick it is a very clearly 

 defined structure. 



Fate of the Germ-Layers 



From the three germ-layers, whose origins 

 have been briefly discussed, are derived all of 

 the organs of the body. 



