562 TEXT-BOOK OF ENTOMOLOGY 



and hind U-shaped rudiment are directed toward each other, and 

 grow towards each other until they meet, and are fused together. 

 Thus the endodermal rudiments arising out of the fusion of the two 

 U-shaped rudiments form two stripes extending along the primitive 

 band and situated mostly under the primitive segments. At the 

 two ends the endodermal rudiment fuses with the stomodeal and 

 proctodeal invaginations. These lateral endodermal streaks now 

 spread out, and gradually begin to grow over the yolk, on whose 

 outer surface they lie. This overgrowth makes the greater advance 

 on the ventral side, so that the two endodermal streaks first unite 

 in the ventral median line and afterward in the dorsal. The yolk 

 in this way passes completely into the interior of the rudiment of 

 the mid-intestine. 



Kowalevsky has already proved that it is the median parts only 

 of the inner layer which at the two ends of the primitive band 

 become separated as endodermal rudiments through the advance of 

 the stomodeal and proctodeal invaginations : the lateral portions 

 become mesoderm. 



Kowalevsky has compared the germ-layers of insects with those of Sagitta. 

 This comparison is supported by the later researches of Heider and of Wheeler 

 on Coleoptera. (See Korschelt and Heider, p. 809.) 



Relations somewhat different from the common type of formation of germ- 

 layers occur in Hymenoptera. Kowalevsky and also Grass! agree that here 

 also the endoderm originally forms a part of the lower (inner) layer. But the 

 separation of the endoderm from the mesoderm goes on in Apis in such a way 

 that the two ends of the inner layer pass up to the dorsal side of the egg, where 

 the fore and hind rudiments of the endoderm extending along the back of the 

 embryo grow together. When the two horseshoe-shaped rudiments have met 

 each other and become fused, the enclosing of the yolk begins, which accord- 

 ingly here proceeds from the dorsal towards the ventral side, instead of vice 

 versa. As a result the endodermal cell-layer in Apis (and also Chalicodoma) 

 at first does not lie under the primitive band, but on the dorsal side of the 

 egg under that flat epithelium, which, arising from the amnion-fold, completes 

 the provisional closure of the back. 



The yolk-cells and secondary yolk-segmentation are discussed by Korschelt 

 and Heider at this point. The yolk-cells are elements scattered throughout the 

 yolk and which partly remain in the yolk during the formation of the blastoderm 

 (Fig. 507, C and Z>), but which in part through a later immigration pass out 

 of the blastoderm into the yolk. Graber has proved the fact of the migration 

 of cells from the lower layer into the yolk, and his observations have been con- 

 firmed by other authors. Indeed, in certain cases (Melolontha), these later 

 immigrant cells are clearly distinguishable by their histological characters from 

 those originally found in the yolk. 



The yolk-cells are regularly scattered throughout the yolk. Their use to the 

 embryo lies in the fact that they absorb the particles of yolk, which they digest 

 and thus reduce to a fluid condition. It usually happens that after the com- 

 plete formation of the primitive band there results a delimitation of the areas 

 enclosing each yolk-cell, and this occurrence is called secondary yolk-division. 



