316 INSECTA. 



The formation of the germ-layers in the Hymenoptera seems to 

 deviate somewhat from the common type. Kowalevsky and Grassi 

 (No. 32),* indeed, agree that here also the entoderm originally forms 

 a part of the lower layer. But the separation of the entoderm from 

 the mesoderm in Apis takes place in such a way that the two ends 

 of the lower layer bend over the dorsal side of the egg, and the 

 anterior and posterior entoderm-rudiments which have thus come 

 to lie on the dorsal side grow towards one another. When the 

 two rudiments, which here also are horseshoe -shaped, have met 

 and fused, the circumcrescence of the food-yolk begins ; in this 

 case the process thus starts from the dorsal side and is completed 

 on the ventral side. It results from this that the layer of entoderm- 

 cells in Apis at first does not lie below the germ-band, but on the 

 dorsal side of the egg below that flattened epithelium which, arising 

 from the amniotic fold, provisionally completes the dorsal surface 

 (p. 287). 



The condition of the entoderm-rudiment in Chulicodoma is somewhat similar 

 (Carriere, No. 13). Here also the entoderm -bands do not lie below the germ- 

 band, but extend beyond the latter towards the dorsal side of the egg. As to 

 the first separation of the germ-bands, Carriere arrived at views approaching 

 those just described, but still revealing in the case of Chalicodoma a peculiar 

 type. The middle plate («?), which becomes invaginated by the formation 

 of the gastrula-furrow, and which, like the lateral plates, shows signs of 

 segmentation at an early stage, is here said to yield the mesoderm exclusively, 

 while the anterior and posterior entoderm-rudiments (ve and lie) arise from 

 a growing zone closely succeeding the middle plate, in the region of which the 

 separation of the mass of entoderm-cells by delamination from the superficial 

 cell -layer which remains in continuity with the ectoderm takes place. 



We have still to mention the yolk-cells and the secondary cleavage 

 of the yolk. The yolk-cells are elements scattered in the food-yolk, 

 some being cells which remained in the yolk at the time when the 

 blastoderm formed (Fig. 131 O and D, z, p. 265), and seme having 

 reached the yolk by subsequent immigration from the blastoderm 

 and its derivatives. Graber first pointed out the immigration of 

 cells from the lower layer into the yolk, and his observations have 

 been confirmed by other authors. In individual cases, indeed (e.g., 



* GRASSl's researches mark a turning-point in the conception of the formation 

 of the germ-layers in the Insecta. It must be recorded to his credit that he 

 was the first to oppose the universal opinion of the time that the yolk-cells 

 represented the actual entoderm of the Insecta, and to prove that the entoderm 

 is a part of the lower layer. The presence of an anterior and a posterior 

 entoderm-rudiment was also correctly made out by him. His views were 

 adopted only later by Kowalevsky (No. 49) and Heider (No. 37), though 

 it should be pointed out that the views put forward by Kowalevsky in his first 

 treatise nearly coincided with what is now known to be the actual condition. 



