140 



CRUSTACEA. 



opposed this view (Xo. 85), maintaining that the yolk-cells, whicli, further, 

 immigrate not merely from the germ-disc, but from the whole circle of the blas- 

 toderm, take no part in the further structure of the embryo, but merely function, 

 as in Mt/sis and Ligia, as vitellopJtags. According to Nusbaum, the actual 

 entoderm in Oniscus originally lies, united with the elements of the mesoderm, 

 in tlie inner layer of the germ-disc, soon, however, becoming arranged into 

 two lateral masses of cells, which fuse and form the mid-gut rudiment, as well 

 as the hepatic diverticula (Fig. 71 B, li). In Oniscus, the two primary hepatic 

 diverticula seem to develop before the mid-gut ; a band-like entoderm -rudiment 

 .starts from the cell-masses above mentioned, and bends round on each side to 

 form a liepatic diverticulum ; these two unite at a later stage, when a short 

 mid-gut develops. In Porcellio, on the contrary, according to Reinhard 

 (No. 91), the mid-gut forms before the hepatic diverticula become marked 

 off. The two jiriniarj' hepatic diverticula divide later into four by longitudinal 

 constriction. It should be noted that the mid-gut in Oniscus, as generally 

 in the higher Crustacea, is restricted to a very short tract near the openings of 



^9 Tit 



Fig. 71. — Two longitudinal sections through the embryo of Oniscus miirarius (after Bobretzky, 

 from Balfour's Text-book). A, younger, B, older stage, do, dorsal organ; hy, food-yolk 

 with yolk-cells; ht, rudiment of the heart; li, liver; m, mb, mesoderm; ol, upper lip; 

 pr, proctodaeuw ; sg, brain ; st, stomodaeum ; vg, ventral nerve-cord ; zp, rudiment of the 

 gastric teeth. 



tlie hepatic diverticula, the larger portion of the alimentary canal arising from 

 the stomodaeum and the proctodaeum. During these processes of development, 

 the layer of mesoderm-cells spreads regularly below the lengthening germ-band, 

 and enters into the rudiments of the limbs. It must be mentioned, however, 

 that according to Wasiljeff (see Grosglik, No. 84), the mesoderm, m Oniscus, 

 breaks up into somites. 



Considerable agreement will be found in this description of the formation 

 of the germ-layers in Ligia and Oniscus, which rests chiefly on Nusbaum's 

 observations, and is supported by Bullar's work on Cymothoa (No. 81), 

 and the results obtained by Pereyaslawzewa (Nos. 70 and 71) and Ros- 

 siJSKAYA (Nos. T2. and 73) with the eggs of various Amphipoda {Gammarus 

 poecilurus, Orclicstia, Caprella, Sunamphinwe, Amjjhitlwe). The elements 

 of the entoderm here arise from an immigration of separate blastomeres 

 into the deeper layers ; these, after dispersing temporarily in the food-yolk, 

 soon become arranged into two lateral entoderm-bands, which first appear in 

 the anterior part of the embryo and join to form the tube of the mid-gut. 

 The primary hepatic diverticula form as outgrowths of the mid-gut, the two 



