136 



CRUSTACEA. 



blind end of the hind-gut (Fig. 63 B, ep), the development of the 

 definitive wall of the mid-gut proceeds in the Decapoda (c/. below, 

 p. 174). In Palaemon, on the other hand, it Avas observed that 

 not all the entoderm-cells to be found in the food-j'olk rise to its 

 surface, to take part in the formation of the epithelium of the 

 mid-gut, but that individual cells remain within the yolk and are 

 later absorbed. ^Ve thus see here the beginning of a difterentiation 

 of the entoderm into two parts, a plastic portion taking part later 

 in the development of the mesenteron and a transitory portion, the 

 cells of which function solely as vitellophags. A similar condition 

 of the entoderm will be found repeatedly in other animals, especially 

 among the Insecta. 



Fig. 67. — Surface view of the egg 

 of Hgia oceanica, at the stage in 

 which the germ-layer.s appear (after 

 Xusbacm). en, point of ingrowth 

 of the entoderm ; k, blastoderin- 

 nuclei ; m, paired points of in- 

 growth of the mesoderm. 



Fig. 68. — Two transverse sections through the germ- 

 disc of Ligia oceanica (after Nusbaum). A, trans- 

 verse section through the anterior region, on the 

 level ab in Fig. 67. B, transverse section through 

 the posterior region on the level cd, Fig. 67. ri, 

 yolk-cells ; ec, ectoderm ; eit, entoderm ; m, meso- 

 derm ; c, central depression of the germ-disc 

 (blastopore). 



F. Arthrostraca and Cumacea. 



Nusbaum's observations on the development of the Mysidae are 

 closely connected with his studies of Ligia oceanica (Xo. 8oa), and 

 afford to a certain extent a key to the comprehension of the formation 

 of the germ-layers in the former group. Here also, after the 

 development of the blastoderm is completed, a thickening is found 

 corresponding to the future ventral side ; this is the germ-disc, 

 which soon shows signs of breaking up into three parts (Fig. 67). 

 The two anterior paired portions represent the areas of formation 

 of the mesoderm (Fig. 67, m), while the posterior unpaired 

 thickening {en), in which, especially at the centre, very active 



