112 



CRUSTACEA. 



This circumcrescence has been described as epibolic gastrulation (Lang), but 

 it must still be considered doubtful whether this is the correct interpretation. 

 According to Nassonow's drawings (No. 13), it appears that when the formation 

 of the blastoderm is completed, cell-elements are ejected by the central food-yolk 

 sphere also, and become massed more superficially near the point last affected 

 by the circumcrescence of the blastoderm (blastopore, Lang, Nassonow). Even 

 this may, perhaps, be only a modification of a primarily complete and later 

 superficial cleavage. The gastrula- stage would have to be sought later, when 

 a small depression of the surface appears (Fig. 59 B, blp, p. 126) at the above- 

 mentioned point, and a simultaneous immigration of the entoderm-cells (en) 

 into the mass of food-yolk takes place. This method of formation of blastoderm 

 would belong to that order of superficial cleavage in which the blastoderm 

 originally appears as a disc, but would be distinguished from the typical 

 examples of that method by the fact that the point at which the disc appears 

 here lies opposite to the blastopore, while in other cases the two points agree in 

 position (cf. p. 115).* 



Fig. 53.— Diagram of the cleavage of Calliunassa subterranea (after Mereschkowski). 

 stages F-H, the foocl-yolk is limited to the central portion of the egg. 



In the 



Cleavage takes place somewhat differently in Sacculina (Van Beneden, 

 No. 25 ; Kossmann). In this egg, the formative and the nutritive yolk become 

 separated only in the four-celled stage which is reached by total and regular 

 cleavage. We then have four micromeres consisting of formative yolk and four 

 macromeres consisting of food-yolk. Whilst the micromeres increase by fission 

 and cover the surface of the egg with a blastoderm-layer, the macromeres fuse 

 to form a simple central mass of food-yolk. The cleavage of the yolk, which 



* [According to Groom (No. I., App. to Lit. Cirripcdia), the smaller of the 

 two cells resulting from the first cleavage, which he terms the first blastomere, 

 forms only a portion of the ectoderm. The second blastomere is formed from 

 the yolk, and the blastoderm is formed partially from the bias torn erea already 

 present, but largely also from fresh blastomeres (merocytcs) yielded by the yolk. 

 The firsl division consequently does not divide the egg into ectoderm and 

 entoderm, but simply into a macromere (yolk-cell) and a micromere, the latter 

 being but one of the future blastomeres, of which the macromere gives oil' a 

 number. He further concludes that the gastrula is formed bv epiboly, and 

 thut there is no superficial cleavage during the later period of division. — Ed.] 



