CEANQON VULGARIS. 109 



of karyokinesis being witnessed in decapod segmentation, 

 excepting in Astacus (Reichenbach, '86 ) ; though it occurs 

 in other Crustacean groups (e. #., Cladocera, Copepoda, 

 Grobben, J 79and J 81)- On account of the large size of the 

 nuclei in the eggs of Cancer and Crangon they form es- 

 pecially favorable objects for studies in this direction. 



THE GASTRULA AND GERM LAYERS. 



Owing to the difficulties of following the changes of the 

 cells in the living egg, I have been unable to follow out 

 the phases of gastrulation as clearly as I could wish ; but 

 still my permanent preparations and my sections give a fair 

 idea of the steps. Three of these are shown in figs. 7, 8, 

 and 9. Of these the earlier is 8, which represents the 

 iuvagination as already begun and is taken from an alum- 

 carmine specimen, mounted entire. It shows the germinal 

 area fading out on all sides into the general blastoderm 

 while near the posterior margin of the area the blastopore 

 is seen, the endodermal cells having already sunk beneath 

 the surrounding surface. I am unable to say whether ear- 

 lier these endoderm cells could have been recognized 

 among the others of the germinal area ; but I feel confident 

 that there is no specialization of the mesodermal cells be- 

 fore the formation of the gastrula such as is described by 

 Grobben ('79) in Moina and ('81') Cetochilus. Neither 

 was there the shallow pit seen by Ishikawa in Atyephyra 

 ('85, pp. 411-412) which is subsequently divided into 

 two. 



In the cells which surround the margin of the blastopore 

 (fig. 8), the nuclei are mostly placed iu the distal ends 

 of the elongate cells, while in fig. 7, which represents a 

 slightly later surface view, this feature of the circum-blas- 

 toporal cells is still further emphasized, the inner ends of 

 the cells seeming to run down into the closing blastopore. 

 What interpretation is to be placed on this I do not know. 



