THE DEVELOPMENT OP VERTEBRATA. 221 



from the first stages of segmentation our types begin to 

 differ in development. We must therefore take them each 

 in turn. 



7. Segmentation in Amphioxus. The ovum contains 

 scarcely any yolk, and its follicle remains around it during 

 early development. Fertilization is said to occur always 

 about seven o'clock in the evening, and segmentation to 

 begin an hour later. Although the ovum is a sphere it is 

 possible to define certain directions in it on the analogy of 

 the naming of the earth the polar body marks what is 

 called the animal pole. This is one end of what may be 

 called the axis : at the other is the vegetative pole. 



ABO 



Fig. 111. Sr.OMENTATlON OF OVUM OF AMPHIOXUS. 



(From Willey, after Hatschek). A, 2-celled stage, side view ; B, 8-celled stage, 

 from animal pole ; C, 82-celled stage, side view. 



The plane of separation of the first two cells is a meri- 

 dional plane (fig. Ill, A); so also is the second, which is at 

 right angles to the first. The third is an equatorial plane. 

 By this time there are eight cells, which remain in contact 

 with one another (fig. Ill, B). Two simultaneous meridional 

 planes diagonal to the first ones follow, and then two 

 simultaneous horizontal planes, and thus we have thirty- 

 two cells (fig. Ill, c). Now, at each division the cells tend 

 to round themselves off into spheres and so to fill up more 

 space. The follicle stretches to allow this; but as the 

 number of cells goes on increasing they become flattened 

 where they are in contact, but at the same time they get 

 pushed away from the centre. The result is that this mass 

 of cells tends to take the shape of a hollow sphere ; and 

 this form becomes more definite as the number of the cells 



