120 



CRUSTACEA. 



Cetochilus, like most free-living Copepoda, belongs to our second 

 type of cleavage. At first the cleavage is total, in later stages 

 superficial (cf. p. 109). As early as the thirty-two-cell stage, the 

 transition to the actual blastula- stage commences, the first histo- 

 logical differentiation in the different germinal layers becoming 

 perceptible. At this stage there is found a small segmentation- 

 cavity in which the food-yolk is deposited, and into which the polar 

 bodies also find their way. A similar immigration of the polar 

 bodies was noticed by Weismann and Ischikawa (No. 6) in the 



- en 



Fig. 57.— Four stages in the development of Cetochilus (after Grobben). a, thirty-two-cell 

 stage seen from the vential side. B, later stage, same aspect; all the germinal layers are 

 already distinct. C, longitudinal section of the gastrula-stage. D, ventral aspect of the 

 gastrula-stage, in which the blastopore is closing, en, central entoderm-cell ; gm, mouth 

 of the gastrula (blastopore); m, mesoderm-cell ; sn, lateral entoderm cells; urn, primitive 

 mesoderm-cells ; vn, anterior entoderm-cells. 



summer eggs of Bythotrephes. It is probable that the small cell 

 observed by Urbanowicz in the cleavage-cavity of Cyclops is also 

 to be referred to the polar bodies. 



If we examine the vegetative pole at the thirty -two-cell stage of 

 Cetochilus, a decidedly bilateral arrangement of the blastomeres can 

 be recognised (Fig. 57 A). We find two cells, a larger one (en), 

 distinguished by the coarse granulation of its protoplasm, and a 

 smaller anterior cell (vn). These two cells lie in the median plane, 



