428 MORGAN. [Vol. X. 



germ-ring. This difference is present, so far as I have been 

 able to determine, from the very beginning, and persists, as 

 will be shown through all the later history of the germ-ring. 



Sections, both longitudinal and cross, through the embryo 

 and germ-ring, at i.io p.m., are shown in Figs, 15, A, B, C, D. 

 The first section. A, passes through the longitudinal median 

 plane of the embryo, and shows the extra-embryonic region 

 and germ-ring in front of the embryo. Fig. B passes trans- 

 versely through the apex of the extra-embryonic region, cutting 

 the germ-ring at the two sides. More magnified portions of 

 the germ-ring of these two sections are shown in C and D. 

 We find that the number of cells in the germ-ring has greatly 

 decreased, and that now only two layers are evident in section 

 — the outer continuous into the extra-embryonic region, and 

 the inner tongue of cells beneath. Occasionally, a few cells 

 are found between these two layers. We find that the germ- 

 ring is narrower, and the cells of its upper layer are not so 

 numerous as in the last stage. 



The under layer of cells forming the tongue has about the 

 same extent as in the preceding stage, but has fewer cells. 

 Comparing Figs. C and D, we see at once a decided differ- 

 ence in size of the two. The lateral section, D, contains more 

 cells and a longer tongue of inner cells, and the cells them- 

 selves are higher. 



Three sections are drawn in Figs. \6, A, B, C, from an em- 

 bryo killed at 3 p.m. (Fig. 4, A). The germ-ring in section is 

 smaller than in the last figures, and contains fewer cells. If 

 we compare the amount of material in the germ-ring at this 

 stage with the amount of material in the embryo, we are 

 struck at once with the comparatively small amount in the 

 germ-ring. The embryo is not yet two-thirds its full length 

 and we see how inadequate the material of the germ-ring would 

 be to complete the remaining third. 



Sections through the germ-ring of an embryo at 5.30 p.m. 

 (Fig. 5, A) are shown in Figs. 17, A, B, C. The first of these 

 passes through the open yolk exposure, and the germ-ring is 

 cut on each side. Fig. ^ is a more magnified portion of 

 the same, and shows how extremely thin the germ-ring has 



