584 MORGAN AND HAZEN [Vol. XVI. 



a change in the character of the cells themselves. The ante- 

 rior end of the archenteron becomes larger, and at the same 

 time the cells covering this portion of the cavity become 

 smaller (PI. XXXIII, Fig. 7 A). This region was at first sur- 

 rounded by the large dark yolk-bearing cells (PI. XXXIII, 

 Fig. 5), but these cells seem to shift posteriorly ; at any rate, 

 the large yolk-bearing cells are now found on the ventral and 

 ventro-lateral walls of the archenteron at the posterior end of 

 the body. 



About this same stage, or a little later, a curious change 

 takes place that has greatly puzzled us. Yolk granules begin 

 to appear in all the cells of the body, and what is most sur- 

 prising is that the yolk in the large endodermal cells does not 

 seem to decrease in amount. We cannot offer, therefore, any 

 explanation as to the meaning of this phenomenon. It might 

 seem that the change was brought about by a decrease in the 

 volume of the cells, and that they contained the same absolute 

 amount of yolk as before, the change being apparent rather 

 than real. Measurements of the embryo show, however, that 

 it is increasing in size, and the body-walls are as thick or 

 thicker than before. The cells are, it is true, higher and nar- 

 rower than before, but we do not see how this alone could pro- 

 duce the effect. However, even in the cells of the ectoderm 

 yolk granules reappear in later stages. A longitudinal section 

 of an embryo at 11 a.m. is drawn in PI. XXXIII, Fig. 7 A. 

 A detailed and more magnified part of the dorsal wall (at ,r, 

 in Fig. 7 A) is shown in Fig. 7 B. The ectoderm that forms 

 the nerve plate contains more yolk granules than in the earlier 

 stages, and the endoderm of the dorsal wall {e?id.) is now also 

 filled with yolk granules. Another part of the same embryo 

 anterior to the last (at j/ in Fig. 7 A) is shown in Fig. 7 C. 

 In this region the ectoderm contains almost no yolk, but the 

 endodermal cells are filled with it, A section through the ven- 

 tral wall (at z in Fig. 7 A) is shown in Fig. 7 D. Here the 

 ectoderm contains very little yolk, while the endodermal cells 

 are tall, closely packed together, and filled with yolk granules. 



A later stage (PI. XXXIII, Fig. 8) shows that the yolk gran- 

 ules reappear even in the ectoderm. At this time the first 



