432 MORGAN. [Vol. X. 



of cells also becomes larger. When the yolk-sphere is covered 

 there are nearly three and a half times as many cells in the 

 extra-embryonic region as in the first stage. What is the 

 source of these new cells .? They may have come from the 

 germ-ring or they may have come by division of the cells them- 

 selves. There is direct evidence, as we have seen, pointing 

 to their origin in part from the germ-ring. On the other hand, 

 if the cells had divided very rapidly during overgrowth they 

 would have become smaller. Cell growth, or simply superficial 

 flattening, might, however, compensate for the reduction in size 

 of the cells if cell-division takes place continuously. Therefore, 

 without a knowledge of how extensively cell-division is taking 

 place we cannot decide this problem. (See Appendix.) 



The bearing of results recorded in the preceding sections 

 may be summed up as follows: 



The most important period in the formation of the embryo 

 seems to be at the time when the embryonic portion (head) is 

 first formed. If it could be shown that the amount of material 

 present at that period in the embryo is equal to the material of 

 the fully formed embryo, there would remain no doubt that the 

 elongation of the embryo was due to axial concentration. 

 In the absence of data we must turn to other sources for 

 information. A study of the method of the formation of the 

 embryo has shown us that the axial concentration must neces- 

 sarily contribute to the elongation of the embryo backwards. 

 A study of the germ-ring points decidedly to the conclusion that 

 one of its functions, at least, is to contribute cells to the extra- 

 embryonic region. Another possible function of the germ- 

 ring is that it may continually bring new cells into the 

 posterior end of the elongating embryo. According to His' 

 conception of concrescence, the sides of the embryo are laid 

 down right and left by the apposition of the germ-ring, so that 

 new material is constantly coming up to form the posterior end 

 of the embryo. We have seen, however, that during the first 

 period of the overgrowth of the germ-ring it is constantly 

 increasing in length, and correspondingly decreasing in breadth. 

 After the equator is passed the reverse process takes place. 

 And during both of these periods there is strong evidence 



