54 



DEVELOPMENT OF THE FROG'S EGG 



[Cii. V 



central nervous system may also grow forward somewhat so that 

 the actual rotation is not great. 



Perhaps the whole process may be made clearer by reference 

 to a series of sections through the egg. These are taken through 

 the meridian that corresponds to the middle plane of the body ; 

 it therefore passes through the upper and lower poles (Fig. 18). 

 The arrows indicate the primary axis. The dorsal lip of the 

 blastopore has formed in Fig. 18, B, and in Fig. 18, C, D, the 

 migration of the lip has gone further over the lower hemisphere. 

 The ventral lip of the blastopore has also formed. Figure 18, 

 D, corresponds to a stage in which the blastoporic circle is com- 

 pleted. In Fig. 18, E, we see that the dorsal lip has travelled 

 further over the lower pole toward the ventral lip. Finally, 



FIG. 19. A. Diagram to illustrate overgrowth of dorsal lip of blastopore. I-IV rep- 

 resent different stages. B. Diagram of cross-section through Z-Y of A, to show 

 lateral lips of blastopore, and mesoderm (M), aud (IW) inner wall of archenteric 

 pit. OW. Outer wall. 



in Fig. 18, F, the egg is represented as having rotated as a 

 whole to bring the embryonic portion above. 



The changes that take place during the closure of the blasto- 

 pore are perhaps more clearly shown by the following experi- 

 ment. By means of a fine pointed needle it is easy to puncture 

 the egg slightly at any given point. When the outer surface 

 of the egg is pierced, there follows a protrusion of material as 

 soon as the needle is withdrawn. At other times when the 

 surface is only indented (not pierced) by the needle, there 

 follows a blunt protrusion of material and the surface remains 

 unbroken. In the latter case the marks do not always last as 



