124 



CRUSTACEA. 



mentioned, shift inwards and lie upon the entoderm (cf. below, 

 Fig. 72 A, g, p. 147). GROBBEN holds that the point at which the 

 blastopore closes corresponds to the future oesophageal invagination. 

 It would, however, be more in agreement with the conditions met 

 with in other Crustacea, especially in the Decapoda, if we might 

 assume that it lay in the neighbourhood of the future anal aperture. 

 While the embryo lengthens, the Nauplius limbs grow out and 

 the rudiment of the brain becomes distinct at the anterior end of 

 the dorsal surface as an ectodermal thickening (neural plate, Fig. 

 72 A, p. 147); the rudiments of the internal organs also undergo 

 corresponding development. The entoderm (en) develops into a 

 cylindrical body, the cells of which, in cross section, appear radially 

 arranged ; no lumen is, however, at first to be seen. The stomodaeum 

 and proctodaeum (Fig. 72 J3, m, a/) arise as ectodermal invaginations ; 

 the former is distinct even in the Nauplius, the latter only at a 



FIG. 58. Three stages in the development of the summer egg of Moina (after GROBBEN). 

 A, egg in the thirty-two-cell stage seen from the vegetative pole. B, blastula stage, same 

 aspect. C, median section of gastrula stage, b, blastopore ; en, endoderm-cells ; g, genital 

 cells ; mx, mesoderm-cells ; n, food-yolk ; s, neural plate. 



later stage. They become connected with the rudiment of the 

 mid-gut. The mesoderm (ms) has spread over the whole ventral 

 surface and has extended anteriorly until it comes to underlie the 

 neural plate. It is situated on either side of the rudimentary 

 intestine and is bilaterally symmetrical ; there is, however, no 

 complete separation of the two parts of the mesoderm-band. The 

 genital rudiment divides to form a paired mass of cells lying on 

 either side of the intestinal canal. 



The food-yolk originally lies in the primary body-cavity. It is 

 absorbed in proportion as the inner organs fill that cavity. In later 

 stages, a few cells separate from the mesoderm and penetrate the 

 food-yolk. They then come to lie on the dorsal side of the embryo 

 and become the fat-body in the adult. 



