428 Avery E. Lambert. 



after they have left the region of the caudal thickening. In Fig. 

 15 some of these cells are seen to be passing into the yolk. This 

 indicates the true origin of the so-called yolk-cells (y-c). Here they 

 increase in size by absorbing nourishment from the yolk, and form 

 the anlage of the endoderm. 



Thus at tlie time when the ventral plate of Balfour — or as I shall 

 call it, the ''germ band" (g.bd.) — is fully established, the three germ 

 layers may be recognized in the embryo ; the ectoderm, which con- 

 sists of the cells of the blastodisc, and their immediate derivatives, 

 which lie on the surface of the egg ; the mesoderm, consisting of cells 

 which have arisen in the region of the blastopore, and which lie 

 immediately beneath the ectoderm ; the endoderm, which is repre- 

 sented by a number of scattered cells that have arisen coincidently 

 with the cells of the mesoderm, and which have migrated into the 

 yolk. 



I can find no evidence that the middle layer is augmented by the 

 addition of cells from the outer layer (Schimkewitsch and Balfour) 

 further than has been stated. Whatever mitotic figures have been found 

 in the cells of the blastodisc, outside of the region of the blastopore, 

 their position indicates that division takes place in the cells of this 

 layer in radial planes. Fig. 14; n.sp. Issue must also be taken 

 with Balfour's statement that the endoderm also contributes to the 

 foiTuation of the mesoderm. As later writers have shown, and as my 

 own results indicate, the endoderm cells do not appear in the yolk 

 until after the cells of the mesoderm have commenced to arise ; the 

 cells of both layers being derived from the same point, namely the 

 proliferating area of the caudal thickening; the cells which pass into 

 the yolk being thrust into that position by the conditions attending 

 their formation, after which they take upon themselves the character 

 of endoderm cells. 



5. 21ie Ventral Plate. — The germ band continues to increase in 

 length by the rapid multiplication of its cellular elements until it 

 covers more than two thirds of the circumference of the egg, thus 

 forming the ventral plate. The caudal and cephalic ends of the plate 

 ultimately come quite near one another on the dorsal surface of the 

 egg. Fig. 9. 



