CLEAVAGE AND FORMATION OF THE BLASTODERM. 



117 



deposit of cells, which are here deeper and form a rounded thicken- 

 ing (germinal disc). 



Not only the eggs of ' My sis and Cuma, but those of several Isopoda are said 

 to show this type of segmentation (Oniscus, according to BOBRETZKY, No. 80, 

 Ligia, according to VAN BENEDEN, No. 1). It is also said by VAN BENEDEN 

 and BESSELS (Ne. 2) to occur in many parasitic Copepoda (Anchorella, Caligus, 

 Clavella, Lcrnaea, Lernaeopoda, Brachiella, etc.). It must, however, be con- 

 sidered probable that the greater number of these cases really belong to 

 Type III. b. This view is supported by the observations of BOUTCHINSKY 

 (No. 37a), who, according to the plates given in his Russian treatise, observed 

 a simple superficial cleavage in Paradopsis cornula. Our retention for the 

 present of the discoidal type of cleavage for the Crustacea is due entirely to 

 NUSBAUM'S recent description of Ligia oceanica, according to which a type of 

 segmentation agreeing with that given above for My sis actually occurs (No. 85a). 



The type of discoidal cleavage here described shows some superficial resem- 

 blance to that kind of discoidal cleavage which, in many groups of animals (e.g., 

 Cephalopoda), is developed from a total, unequal cleavage. Closer examination 

 reveals, however, that what we are now 

 considering is a peculiar process of dis- A 



coidal development of the germ, which 

 has evidently developed independently 

 among the Crustacea out of a superficial 

 type of segmentation. For where dis- 

 coidal cleavage has developed out of 

 total, unequal cleavage, we find that the 

 pole of formation of the germ-disc 

 corresponds to the animal pole, its 

 gradually out-spreading edge to the 

 blastopore, and the plug of yolk to 

 the vegetative pole of the egg. Here, 

 however, in the discoidal cleavage of 

 the Crustacea, it is quite otherwise. 

 The formative pole of the germinal 

 disc corresponds to the ventral side of 

 the embryo, and all observations point 

 to the fact that here also the formation 

 of the germ-layers commences, and the 

 important but somewhat obscured process 

 of gastrulation. The circumcrescence 



of the food-yolk here proceeds from the ventral to the dorsal side, and has 

 evidently in this case nothing to do with gastrulation, since we should otherwise 

 be compelled to assume, in Crustacea with discoidal cleavage, a blastopore 

 closing dorsally, which would be in contradiction to the conditions found in 

 other Crustacea. 



By the above considerations we are led to regard the discoidal cleavage of the 

 Crustacea as an extreme case of that type of cleavage described above as 

 Type III. 6. In the discoidal method, the formation of blastoderm at the pole 

 where it originates is carried out so early, that its rudiment originally consists 

 of a single blastoderm-cell, which, by subsequent successive division, yields the 

 whole blastoderm. 



FIG. 56. Two stages in the segmentation 

 of Mysis (after VAN BENEDEN) as an 

 example of discoidal cleavage. A, two 

 cells are to be seen at the surface of the 

 yolk. B, the two cells have increased by 

 division and form a cap. 



