No. 2.] FORMATION OF THE FISH EMBRYO. 429 



become. Fig. C cuts the germ-ring very near to the embryo, 

 and here we find quite an accumulation of material — relatively 

 a very large amount. 



Finally, in Figs. 18, A, B, we have drawn to the same scale 

 as the preceding figures, sections through the germ-ring of 

 embryos killed at 7.05 p.m. (Fig. 6, A). The first section is from 

 a median longitudinal section of the embryo, and cuts the 

 germ-ring at a point exactly opposite to the embryo. The 

 germ-ring of this figure {A) should be compared with the germ- 

 ring of Fig. 14, C, through the corresponding region. Fig. 

 18, B, is from a cross-section of the germ-ring at the side (not 

 near to the embryo). These sections show us to what a small 

 structure, quantitatively, the germ-ring has been reduced dur- 

 ing overgrowth of the yolk, and what a few cells it contains 

 after the process. The apparent meaning of this change will 

 be more fully discussed below. 



From a series of surface preparations of the extra-embryonic 

 region, I have attempted to discover what part the cells cover- 

 ing this region play in the process of overgrowth. 



I hoped that such a study might give some clue as to the 

 part taken by the germ-ring in the process. If the number of 

 cells over this region increased in number at a greater rate 

 than their own processes of cell-division would account for, then 

 the new cells added must come in from the germ-ring. If, on 

 the other hand, the number of these cells remained constant 

 throughout, then the germ-ring could take no part in the for- 

 mation of the extra-embryonic region, and the germ-ring must 

 ultimately pass entirely into the embryo. I have worked on 

 material prepared by osmic acid (followed by Merkel), but 

 these preparations have not given sufficient data in regard 

 to karyokinetic processes, that may go on in the extra-embryonic 

 region. It is important to determine the latter point, and I 

 regret that my material has not been adequate to do it. (See 

 Appendix.) 



In the following figures the outlines of the cells figured cor- 

 respond to the second layer of surface cells of the earlier stage, 

 since the outer, very much flattened layer (covering-layer) of 

 the earlier stages becomes very thin during the later stages, 



