74 OUTLINES OF CHORDATE DEVELOPMENT 



toward the attraction sphere (Fig. 26, A). After synizesis the 

 chromosomes scatter through the nucleus as small feathery 

 bodies (Fig. 26, C), which stain lightly and become vacuolated, 

 finally losing their identity. Meanwhile small yolk particles 

 of mitochondrial nature appear in the cytoplasm, in the region 

 of the attraction sphere and apparently under its influence. 

 During the growth period the mitochondrial particles and 

 yolk bodies accumulate rapidly, especially around the attrac- 

 tion sphere giving it the appearance of a yolk nucleus, whence 

 they extend to other parts of the cell except in the region 

 immediately surrounding the nucleus. Finally the "yolk 

 nucleus" breaks down and the deutoplasm around it scatters 

 through the cytoplasm (Fig. 26, D, E}. 



Toward the close of this process the nucleus moves toward 

 one side of the cell, marking the polarity of the ovum, and 

 from the first the yolk accumulates in the side opposite the 

 nucleus or vegetal region, while around the nucleus in the 

 animal region the protoplasm contains much less deutoplasm; 

 the superficial protoplasm of the animal pole also contains 

 many pigment granules. 



As the growth period of the primary oocyte is completed 

 the nucleus moves up to the surface of the cell which becomes 

 flattened or even depressed toward it; at the same time the 

 pigment over the nucleus is partially displaced or withdrawn 

 and the lighter fovea results. The nucleus is very large and 

 clear; no chromatin network is visible and the only chromatic 

 bodies in it are the nucleoli. The nucleoli are quite numerous 

 and apparently of two kinds true nucleoli whose real nature 

 is doubtful, and chromatin nucleoli. The nucleus becomes 

 elongated parallel with the surface of the egg and pigment 

 accumulates around it, while the nucleoli become vacuolated 

 and much enlarged. Some of the nucleoli dissolve while 

 others fuse into large masses and the chromatin nucleoli collect 

 in a small group near the center of the nucleus. 



While these events are occurring within the nucleus a pro- 

 nounced cytoplasmic modification has appeared. Along the 

 lower or inner side of the nucleus a small cytoplasmic area has 



