EMBRYOLOGY 387 



inert yolk material. The result is that the number of small dark 

 cells is greater than that of the light-coloured cells. 



When the eight-celled stage is reached, each cell rounds off 

 internally so as to leave a small internal space, the beginning of the 

 blastoccel or segmentation cavity, which right from its commence- 

 ment is eccentric in position lying nearer the animal pole. This 

 increases in size arid is quite well marked when there are from 32-64 

 cells present, and so the embryo as a whole is now a blastula whose 

 wall is much thinner at the animal pole than elsewhere. Even 

 by the time the sixty-four-celled stage has been reached, we find a 

 departure from the procedure found in Amphioxus, for some of the 

 cells have divided in a plane tangential to the surface in such a manner 

 that one of the daughter cells comes to lie on the inside. Thus it 

 is that all divisions are not marked on the outside and the wall of 

 the blastula, instead of consisting of a single layer of cells, comes to 

 consist of several layers. The fully formed blastula is spherical 

 and slightly larger than the ovum and the symmetry indicated in 

 the egg has become more obvious. The anterior wall of the segmen- 

 tation cavity is a little thicker than the hinder while the pigmented 

 cells extend a little further down on the posterior side. Also the 

 small cells at the animal pole have become differentiated into an 

 outer compact superficial or epidermal layer and an underlying 

 more irregular and looser "nervous layer." The external area of 

 pigmentation has also spread out so that it covers more of the 

 outside of the blastula than it did of the ovum. 



The process of forming a double layered embryo or gastrula 

 is very different from that in Amphioxus. The first indication of 

 the change is the appearance of a slight irregular horizontal groove 

 on the posterior side of the blastula. This soon assumes a crescentic 

 shape and becomes more strongly marked as the cells on its upper or 

 convex border are deeply pigmented while those on the lower edge 

 are white. A section through the embryo at this stage reveals the 

 meaning of the crescent. It is due to a double process ; in the 

 first place the cells of the animal pole are actually growing downwards 

 at the point to enclose the vegetative cells and these in their turn 

 are arching up into the animal hemisphere. The edge of the crescent 

 extends sideways further and further over the vegetative cells until 

 finally its edges meet and it forms a slightly oval area. This is the 

 blastopore, and owing to its position with regard to the orientation 

 of the embryo as a whole, we are able to see now that the original 

 point at which it started was the dorsal lip, and so the ventral lip 

 is the last to be formed. The blastopore when first completed lies 

 on the equator at the posterior end, but as development proceeds 

 it becomes smaller and smaller, and the centre of gravity of the embryo 



