134 CRUSTACEA. 



formation* in the entoderm-cells of the ventral wall of the gastrnla- vesicle. 

 These secondary mesoderm- cells disappear later, and REICHENBACH holds that 

 they are transformed into blood-corpuscles. A similar secondary mesoderm has 

 been observed by ISOHIKAWA in Atyephyra, and by LEBEDINSKY in Eriphia, 

 and further by HERRI CK in Alpheus and ffomarus. 



E. Schizopoda. 



The formation of the germ-layers in the Schizopoda is best illus- 

 trated by the observations of J. NUSBAUM on Mysis chamaeleo (Nos. 

 38 and 39). Cleavage is here discoidal (cf. p. 116). The first 

 cleavage-nucleus attains an entirely superficial position, and yields, 

 by division, a rounded germ-disc, corresponding in position to the 

 ventral side of the egg and the later posterior end of the body. 

 Even from the earliest stages, the germ -disc can be seen to be 

 composed of two layers. The more superficial of these two layers 

 spreads out more and more, finally covering the whole egg as a 

 delicate blastoderm, while the cells of the inner layer pass into 

 the food-yolk, become distributed within it, and are an essential 

 factor in its disintegration, engulfing the yolk and digesting it like 

 amoebae. These so-called vitellopliags are commonly said to take 

 no part in the later formation of the embryo ; it is, however, possible 

 that, at a later stage, they form blood-corpuscles. 



After the complete development of the blastoderm, the position 

 of the former germ-disc is still indicated by a thickening which 

 soon differentiates into three lobes. Two lateral lobes grow out 

 anteriorly to form the lateral paired halves of the germ-band, while 

 the unpaired, median and somewhat posterior lobe must be con- 

 sidered as the caudal or abdominal rudiment (Fig. 77 A, p. 153). 

 In this latter region, below a transverse furrow which must doubtless 

 be referred to an ingrowing caudal fold, the entoderm arises by 

 delamination from the cells of the inner layer. The mesoderm, on 

 the contrary, is said to arise from the ectoderm along the whole 

 length of the paired lateral halves of the germ-band, either by the 

 division of individual blastomeres, the inner portions of which shift 

 into the mesoderm-layer, or else by the immigration of complete 

 blastomeres. In the Nauplius stage, the mesoderm-layer thus formed 

 shows not only a distinct arrangement into paired mesoderm-bands, 

 but a division of the same into separate segments; the formation 

 of coelomic sacs, on the other hand, has never been observed. 



* [Recent research on cell-division has shown that most of the cases in which 

 cells are said to arise by endogenous cell-formation can be differently interpreted. 

 The whole theory of endogenous cell-formation is indeed very improbable. ED.] 



