512 KNOWER. [Vol. XVI. 



and those changes in the blastoderm cells which result in the 

 formation of the embryonic rudiment on the surface of the 

 yolk. 



I have endeavored to answer the following questions : Is the 

 disc formed immediately during the segmentation, by cells wan- 

 dering directly to the point on the surface where the disc is 

 to appear .-' Is a blastoderm over the entire yolk surface first 

 formed as a result of segmentation, and then the disc from its 

 cells } Again, if this latter is the method, is the embryo a 

 result of a simple multiplication of the cells of a restricted area 

 of the blastoderm, or is there some other factor present in the 

 formation of the disc } 



The answer to these questions requires considerable atten- 

 tion to the earliest stages, but is of much importance to an 

 understanding of the origin of the " under-layer " and of the 

 amnion. 



The position of the polar-bodies marks the dorsal pole of the 

 shorter axis of the egg. I have not studied the formation of 

 the polar-bodies from the nucleus, as the few eggs which were 

 at the proper stage did not show the process distinctly enough. 

 To separate from the polar-bodies, the segmentation nucleus 

 moves from the center of the yolk to the center of the dorsal 

 surface. After their formation, the polar-bodies appear as two 

 little rod-like masses of chromatin surrounded by a small quan- 

 tity of protoplasm, and lie at about the middle point of the 

 flattened dorsal surface of the yolk-mass (PI. XXIX, Figs. 2 

 and 3). Later the chromatin breaks into fragments, but the 

 little collection remains visible for a number of divisions. 



The segmentation nucleus, on returning from the dorsal 

 surface, lies, just previous to the first division, in the center of 

 the yolk at the intersection of the shorter and longer axes of 

 the ^zg (PI. XXIX, Fig. 2). 



The first spindle lies at right angles to the shorter axis, so 

 that one of the cells arising from the first division wanders 

 toward the enlarged posterior pole, where the embryo will first 

 appear. The other cell remains near the position that the 

 mother nucleus held (PI. XXIX, Fig. 3). At the start, then, there 

 is a decided proliferation toward the future embryonic area. 



