8 



INTRODUCTORY 



crescent becomes dorsal, the opposite side (on which the 

 spermatozoon had entered) ventral, while the anterior end is 

 just above the animal pole, the posterior end therefore just 

 below the vegetative pole. 



Segmentation now occurs, the divisions of the first few 

 phases passing through the egg-substance in a perfectly 

 regular and definite way, the first being meridional, the 



A 



B 



C D 



FIG. 3. Formation of the grey crescent in the frog's egg (R. tem- 

 poraria). A, B from the side ; c, D from the vegetative pole. In A, C 

 there is no crescent, in B, D a part of the border of the pigmented area 

 has become grey. 



second meridional and at right angles to the first, the third 

 latitudinal, the fourth meridional, the fifth latitudinal. This 

 is the typical sequence, but variations from this radial type 

 are frequent. Subsequent divisions are irregular, tangential 

 divisions occur, and a segmentation cavity is formed. It is 

 noticeable from the very beginning that the animal region of 

 the egg divides more rapidly than the vegetative region, owing 

 to the retarding influence of the yolk. Hence at the end of 

 cleavage the animal hemisphere consists of small cells with 



