430 MORGAN. [Vol. X. 



and the cell-outlines are not apparent in the osmic preparations. 

 (See Appendix.) 



The series of eggs from which the first three figures were 

 taken had been fertilized at 10.30 a.m. The first lot were 

 killed at 6.05 p.m. and are, therefore, 7>^ hrs. old. The sur- 

 face cells of the inner layer of the extra-embryonic region of 

 this stage are shown in Fig. ig, A. The group of cells A is 

 taken from that portion of the blastoderm that is to become 

 the extra-embryonic region. The group marked 19, B, comes 

 from that portion of the surface where the embryo will sub- 

 sequently form. Even at this early stage the difference in size 

 of the cells of the two regions of the blastoderm is very marked. 

 I have not made any attempt to see how far back this difference 

 can be found in the earlier stages. Henneguy says that Kow- 

 alewsky found it quite early, and I have seen that it is present 

 before any other structures indicate where the embryo will form. 



The next two figures, 20, A, B, are taken from a blastoderm 

 at 8.30 P.M., from regions corresponding to those of the last 

 figures. The contrast between the two sorts of cells is now 

 much more marked than in the preceding stages, because the 

 extra-embryonic cells are larger than before, and the embryonic 

 cells are smaller. 



The next figures, 21, A, B, are from an embryo at 9.30 p.m., 

 and the contrast is found to be more marked than in the last, 

 and for the same reasons. At 19 hours after fertilization the 

 cells of the extra-embryonic region are not very much larger 

 than in the last, but the superficial embryonic-cells are decid- 

 edly smaller (Fig. 22). At 20 hours the two sorts of cells are 

 shown in Figs. 23, A, B. In C a few cells are shown from the 

 posterior end of the embryo above the 'tail-knob,' and in D the 

 superficial cells of the second layer of the germ-ring are drawn. 

 The cells of the germ-ring are intermediate in size, between 

 those of the extra-embryonic region and the embryo. Those 

 in the posterior end of the embryo (C) are larger than those 

 in the anterior end (B). This same relation seems to hold 

 throughout the series. 



At 24 hours when the germ-ring is nearly closed, the size of 

 the extra-embryonic cells is shown in Fig. 24. At this stage 



