13G 



rit y in the segmentation, and lack of uniformity in the rate at which 

 this process proceeds in perfectly normal eggs. Some eggs which after- 

 wards develop remain with yolk unsegmented until the third or fourth 

 day after fertilization. The first segmentations affect only the proto- 

 plasm, and take place near the centre of the yolk. The cells thus for- 

 med pass out toward the periphery of the egg, and some of them — 

 3— 4 in number — outstripping the rest , reach a part of the surface 

 first, where they initiate the segmentation of the yolk (Fig. 1). The 



Fig. i. 



Fig. 2. 



Fig. 1. Egg with unsegmented yolk. Nuclei of four cells migrating to one part 

 of the surface, can be seen just below the surface. C = chorion. 



Fig. 2. Profile view of egg as the beginning of yolk segmentation 20 nuclei 

 visible at surface: the yolk on one side is raised into hilloks about the nuclei just 

 under the free portion of the chorion [C], while that of the opposite side is unseg- 

 mented and free of protoplasm. 



nuclei, though not quite at the surface, appear to he so, and the yolk 

 amasses itself about them in the form of hillocks. These constitute 

 the first yolk segments or pyramids. A profile view of the egg shows 

 a perfectly regular contour on one side, while on the other it is thrown 

 into undulations which correspond to the segments just noticed (Fig. 2.). 

 The lobster's egg at this stage strongly recalls the meroblastic type. 

 and would seem to represent a transition between this and the ordinary 

 superficial segmentation of Decapods. This, however, is not quite 

 true, as will be presently seen. 



Cells not only spread over the surface by division but others also 

 reach it by independent migration from the yolk , until , about forty 

 hours after fertilization, the superficial yolk is entirely segmented. 

 About thirty segments are now present. The cells over one half of the 

 egg (those which have reached the surface first) are often preparing to 

 divide by the time the yolk over the opposite side is completely seg- 

 mented. This handicapping of certain cells by others still further tends 



