96 Vertebrate Embryology 



Segmentation of the Egg 



Segmentation begins at about the time the 

 egg enters the uterus, and is usually completed 

 when the egg is laid ; so that the hen's egg is 

 in a more advanced state of development, at 

 the time of laying, than is the frog's egg. 



It will be remembered, in the case of the 

 frog, that, in the process of segmentation, the 



A 



27 



Fig. 34. A. — Yolk with the blastoderm in the 



CENTRE, THE LATTER SHOWING THE FIRST TWO CLEAVAGE 



PLANES. B. — The blastoderm, on a larger scale and 



AT A LATER STAGE OF SKGMENTATION. ThE OUTLINES OF A 

 DOZEN OR MORE CELLS MAY BE SEEN. (After Duval.) 



entire egg was divided by the segmentation 

 planes that passed through it ; but that the 

 blastomeres or segments into which the egg 

 was divided were not all of the same size. 

 Such a case, where the whole egg is divided 

 by the segmentation planes, is known as com- 

 plete or Jioloblastic scgincntLition ; and where, as 



