580 MORGAN AND HAZEN. [Vol. XVI. 



After this stage the blastopore closes in more rapidly from 

 the ventral side, and cell division seems to be more rapid on 

 this side, both in ectoderm and endoderm. 



Distribution of Yolk in the Embryo. 



At the time when the blastula wall first begins to flatten we 

 have attempted to discover whether the yolk is equally distrib- 

 uted throughout the endodermal plate. In many cases we 

 have found that the embryos preserved in Flemming's or in 

 Hermann's fluid show at one end a region of small cells bear- 

 ing fewer yolk granules, and these granules are lighter in color 

 than those of the rest of the inturned cells ^ (PI. XXXIII, 

 Fig. i). At the opposite end of such a section there is an 

 abrupt demarcation between the large yolk-bearing cells that 

 are turned in, and the smaller and lighter cells outside. This 

 difference in the two ends of the section is shown by many 

 series. In other series, cut more or less at right angles to the 

 preceding, the difference is not seen, of course, in the mid- 

 dle sections of the series, but generally at one end of the 

 series the lighter cells can be found, and at the opposite end 

 the yolk-bearing cells extend to the margin of the blastopore. 



This difference in the endoderm we have been able to trace 

 throughout all the subsequent periods of development, and we 

 have been able to show that the region of clearer cells be- 

 comes the dorsal wall of the archenteron. Hence the dorsal 

 lip of the blastopore is identified even at the time when the 

 first beginning of gastrulation is evident. It is, therefore, 

 highly probable that even in the spherical blastula there is a 

 bilateral symmetry already present. We have laid more stress 

 on this means of orientation than on any other, and have deter- 

 mined the dorsal and ventral sides in this way. 



A later stage in the gastrulation is shown in PI. XXXIII, 

 Fig. 2. At this time the endodermal plate has bent upward 

 and almost touches the ectoderm. The section is in a dorso- 



1 It is difficult to determine whether the lighter granules are in reality lighter 

 than those elsewhere, or whether it is only an optical effect of the light passing 

 through fewer granules. 



