76 



DEVELOPMENT OF THE FROG'S EGG [Cm. VII 



contact, the two half-plates are separated in the middle by the 

 diameter of the egg-, but at the anterior and posterior ends the 

 half-plates unite to form the ring. In section, a cord of cells, 

 the notochord, is found beneath each half of the medullary fold; 

 and between the yolk-cells and the ectoderm there is also found 

 a sheet of tissue representing the mesoderm. Hertwig, in 1892, 

 described a large number of these embryos. One is shown in 

 surface view as seen from the white pole, in Fig. 28, A. The 

 embryo is at a later stage of development than that described 

 above. The exposed white yolk, turned toward the observer, 



Fig. 28. — Two " spina-bifida " embryos. (After Hertwig.) 

 stage (different embryos). 



A. Earlier, B. older 



is surrounded by a groove, and outside of the groove there is a 

 bounding darker ridge. In the anterior portion of the white 

 is seen a crescent-shaped depression. A cross-section through 

 the middle of the body of an embryo similar to the last is 

 shown in Fig. 29, A. The exposed yolk is seen at Y. On 

 each side of this there is a depression, and beyond the depres- 

 sion a thickened ridge composed of ectoderm cells. Each ridge 

 passes over on its outer side into the ectoderm that covers 

 all the lower part of the embryo. Even in their present stage 



