248 



Elementary Biology. 



shell, formed by the glandular area of skin before mentioned 

 as occurring, roughly speaking, between the twenty-fourth 

 and thirty- sixth somites, and named the clitellum. 



After a short period of rest, during which, no doubt, 

 molecular rearrangements are taking place in the protoplasm 

 of the embryo, segmentation commences. The result of 

 segmentation is the formation of, what was termed in the 



FIG. 124. PRIMARY STAGES IN THE SEGMENTATION OF THE FERTILISED 

 OVUM OF Lnmbrictts. (Schiifer.) 



preceding section, a blastosphere, but in this case of a 

 peculiar type. In the worm the blastosphere is flattened, and 

 the cells forming the sac-wall are of unequal size, being on 

 one side large and clear, on the other, small and more 

 granular. Further there is nothing corresponding to a seg- 

 mentation-cavity between the two layers. But owing to I he 

 more rapid growth of the smaller cells, the larger clear cells 

 become enclosed, so that the embryo is composed at this 

 stage of an outer epiblast and an inner hypoblast layer en- 

 closing a small cavity. The worm is now in the gastrula 



