Manchester Memoirs, Vol. I. (1906), No. 3. 19 



ingrowth of cells in this region (/^z^. 6, B, C). The yolk, 

 however, is not wholly covered by this process. As soon 

 as the body of the embryo is formed overgrowth ceases to 

 be bilateral ; all parts of the edge of the blastoderm — 

 that is, the lip of the blastopore, now take part in the 

 process, and the blastopore eventually closes at the 

 vegetative pole {^Fig. 6, D). 



It is worth while noticing that in this case, where, 

 since the &g^ is ellipsoid, the egg-axis can always be 

 recognised, there cannot be the least doubt that the 

 movement of the lip of the blastopore really involves an 

 overgrowth, and is not due to a rotation of the egg as a 



ELASMOBRANCHII. 

 The Elasmobranch ovum is megalecithal and mero- 

 blastic. At the end of segmentation there is found at the 

 animal pole a blastoderm lying on and continuous at its 

 edges with the unsegmented yolk. The blastoderm 

 consists of a superficial upper layer of columnar cells 

 arranged in an epithelium and a mass of loosely connected 

 lower layer cells lying in the cavity — the segmentation 

 cavity — which separates the upper layer from the yolk. 

 The lower la}'er cells are disposed principally in two 

 groups ; a group occupying almost the whole vertical extent 

 of the segmentation cavity in the anterior region of the 

 blastoderm, and a smaller group close to the margin of 

 the blastoderm at the posterior end i^Fig. 9, A). Below 

 the segmentation cavity is the yolk, unsegmented but 

 nucleated. Many of these yoke-nuclei are exceedingly 

 large, and a good number are, according to Riickert, de- 

 rivatives of accessory spermatozoa. Though small cells 

 are continually segmented off from the yolk to be added 

 to the lower layer cells it is doubtful whether any of the 

 yolk-nuclei play any part in the formation of the embryo ; 



