126 



CRUSTACEA. 



assuming that two cells lying behind this budding zone, at the edge 

 nearest the terminal segment, were primitive mesoderm-cells ; but 

 Claus proved that these cells, of which there are two on each side, 

 ia the stages hitherto examined, do not take part in the pro- 

 duction of mesodermal elements. The most striking peculiarity in 

 Dranchipus appears to be the early development of the splanchnic 

 mesoblast forming the intestinal muscles. 



C. Cirripedia. 



After tlie l)lastoderm is fully formed (c/. p. Ill), the embryo of Balanus 

 consists of a layer of cells (ectoderm) completely covering the suiface of a 

 central mass of food-yolk (Fig. 59 B). Near the posterior pole of the egg, 

 where an insignificant depression {bl, the blastopore) is perceptible, tlie blasto- 

 derm consists of several layers. The deeper layers represent the rudiment of 



Fig. 59.— Longitudinal sections through three embryonic stages of Balanus improi'isus (after 

 Nassonow). a, Interstage of segmentation (cf. above, Fig. 52, p. 111). B and G, stages in 

 which the germinal layers are being formed, hi, blastopore ; cc, ectoderm ; en, entoderm ; 

 ms, mesoderm. 



the entoderm (en) and the mesoderm {ms). From this point the mesodermal 

 elements are distributed along the ventral side of the egg (Fig. 59 C) in the 

 form of a symmetrical mesodermal plate, the ectoderm being correspondingly 

 thickened, so that in this way a ventral thickening of the superficial layers of 

 the embryo resembling a germ-band arises. The elements of the entoderm, on 

 the contrary, are now evenly distributed througli the food-yolk (Fig. 59 C, en) ; 

 then, by a secondary cleavage of the yolk, the formation of definite cell-areas is 

 brought about. * Finally the nuclei of the yolk-laden entoderm-spheres shii't to 

 the surface to form the epithelium of the mid-gut, whose cavity arises by the 

 absorption of the food-yolk (</. below, p. 174). The details of all tliese processes 

 are still far from clear. Nassonow's text being in Russian, it was only possible 

 to gather tlie most important points from the figures. It appears tliat the blas- 

 topore corresponds in position to the future anal aperture. Tliese stages of 



* [See Giiooji (Ai)p. to Lit., Cirripedia I.). The cleavage of the yolk is not a 

 sudden secondary process, but is comj^lete from an early stage. — Ed.] 



