424 MONTGOMERY. [Vol. XV. 



the primitive yolk ball may break either into two fragments 

 (which are usually unequal in dimensions), each of which then 

 fragments further, or it breaks at once into a considerable 

 number of larger granules. The final stage in this process of 

 division shows the daughter yoke balls fragmenting to form 

 the ultimate yolk spherules (Fig. 118) ; the latter stain an 

 orange red with eosin, are homogeneous in appearance, and 

 usually oval or spherical in form, seldom irregular. Two main 

 stages may accordingly be distinguished in the formation of the 

 yolk : (i) the formation of a large, regularly shaped yolk ball ; 

 and (2) the successive fragmentation of this ball, accompanied 

 by a gradually lessening afifinity for stains, resulting in the 

 evolution of the mature, small yolk spherules, the cytoplasm 

 of the ripe egg being thickly filled with the latter. It is usu- 

 ally the case that the yolk ball attains its greatest size at the 

 end of the first stage. In cells of medium size all the various 

 stages of yolk formation may be found, which shows that the 

 yolk balls are being successively produced and are successively 

 fragmenting ; quite a number of these balls need to be pro- 

 duced in order to furnish the large quantity of yolk globules of 

 the mature egg. The time when the yolk balls first appear, 

 the size they reach, and the manner in which they segment, 

 seem to vary much in individual cells. 



I have not been able to determine the manner of the first 

 differentiation of the yolk substance in the cytoplasm. Two 

 possible explanations suggest themselves : (i) either a certain 

 portion or constituent of the cytoplasm changes into yolk sub- 

 stance ; or (2) the yolk balls may represent a nutritive 

 substance accumulated in the cytoplasm, which may have been 

 derived from the blood or from some neighboring tissue, if not 

 directly from the posterior intestine. But it is without doubt 

 that this substance is not of nuclear origin, for the yolk balls at 

 their first appearance are not in contact with the nucleus, but 

 usually at some distance from it ; and also during the earlier 

 stages of the yolk formation the nucleus is irregular in outline, 

 with short, blunt processes, which would show that it is tak- 

 ing up substances from the cytoplasm, rather than excreting 

 substances. 



