106 THE DEVELOPMENT OP 



marked differences from other genera except in the direc- 

 tion already indicated. Figs. 3 and 4 are respectively 

 surface and sectional views of a stage with about sixteen 

 nuclei, and are introduced for the purpose of showing the 

 external appearance and some of the points of internal 

 structure. As will be seen from fig. 4, most of the pro- 

 toplasm has reached the surface of the egg bat there still 

 remains some near the centre of the yolk. Whether 

 this is the same as the protoplasm described by several 

 authors (Reichenbach in Astacus, Ludwig in Spiders, 

 '76) I cannot say ; but I am certain not only that it is 

 derived from the first segmentation nucleus, but also that 

 it plays a part in the formation of the blastoderm. As 

 this retardation of a portion of the cells in their journey 

 to the surface seems to explain several points in the early 

 stages of the arthropods, a moment may be given to it. 

 While the cells which have reached the surface and which 

 have thus formed a blastoderm are undergoing division, 

 this central protoplasm also divides and gives rise to sev- 

 eral cells which migrate, though much more slowly to the 

 surface. In fig. 6 this migration is clearly shown, and 

 it is to be noted that the cells are all proceeding in the 

 same direction, apparently toward one side of the egg, 

 This is shown in several of my sections, and not one indi- 

 cates that these belated cells migrate to several portions 

 of the surface. 



This migration of the belated cells toward one point, 

 together with a more rapid division of those in the same 

 region which earlier reached the surface, results in the for- 

 mation of the not very clearly delimited germinal area. 

 Fig. 5 represents a section through the area shortly be- 

 fore the formation of the gastrula. In other parts of the 

 egg, the nuclei are placed near the surface as at b, each sur- 

 rounded with a scanty amount of protoplasm ; but in the 



