87 



At the period of the appearance of the first and second merid- 

 ional divisions of the egg the resulting blastomeres appear to be 

 separated in the living egg by complete furrows. Sections show, 

 however, that the line of separation is brought about by vacuoles 

 appearing in the plane of the furrow. Figure la is a drawing of 

 a section of a 4-cell stage of the plaice, and indicates the early con- 

 dition of the vacuoles. Their small number and relatively large 

 size during the early stages are likewise evident from figs. 16 and Ic. 

 It may also be noted that the vacuoles do not open to the surface, 

 nor do they pierce a similar film of cytoplasm which separates the 

 blastomeres from the yolk. 



The subsequent fate of the vacuoles will be evident from figs. 

 1/, g and I. In the case of fig. Ic an appearance is produced which 

 is reminiscent of the well-known figures of Agassiz and Whitman. 

 A thin layer of protoplasm is seen stretching from the marginal cells 

 below the central cells. This is due to the fact, however, that the 

 drawing has been made from a section near to the plane of separation 

 of these cells from their neighbours. The vacuoles next the yolk 

 extend under the blastomeres incompletely separating the blasto- 

 meres from a laj^er of cytoplasm which thus becomes better defined. 

 Similarly the vacuoles expand under the superficial layer of cyto- 

 plasm so as incompletely to separate that layer from the blastomeres 

 below it (figs. Id and Ih). When segmentation has proceeded so 

 far as to bring about two layers of blastomeres, as shown by those 

 figures, the upper layer is attached to a thin covering layer of proto- 

 plasm ; the lower layer is similarly related to a thin protoplasmic 

 sheet which lies between it and the yolk. The blastomeres, 

 however, of both layers are intimately connected with one another 

 laterally and vertically by protoplasmic bridges. 



The continued fragmentation of the nuclei leads to the develop- 

 ment of a large number of blastomeres in the polar mass of proto- 

 plasm, the blastodisc of authors. During the process in the case 

 of the cod and whiting, to a less extent in the case of the plaice, the 

 space occupied by the blastomeres becomes deeper and at the same 

 time narrower. Measurements show that the contraction in the 

 blastodisc of the egg of the whiting is about '09 mm. (c/. fig. 2a, page 

 84). A comparison of figs, la and 1^, which are drawn under the 

 same power, will serve to indicate how little the blastodisc has 

 changed during the process of segmentation in the plaice. The 

 protoplasm of the area is moreover perfectly continuous. All the 



