ii8 



THE GERM-CELLS 



(c) Yolk-nucleus. — The term "yolk-nucleus" has been applied to 

 various bodies or masses that appear in the cytoplasm of the growing 

 ovarian Q.%g ; and it must be said that the word has at present no 

 well-defined meaning. We may distinguish two extreme types of 

 "yolk-nuclei" which are connected by various transitional forms. 

 At one extreme is the yolk-nucleus proper, as originally described by 

 von Wittich ('45) in the eggs of spiders and later by Balbiani ('93) in 



B 



C 



E 



Fig. 60. — Young ovarian eggs of birds and mammals. [Mertens.] 

 A. Egg of young magpie (8 days), surrounded by the follicle and containing germinal vesicle 

 and attraction-sphere. B. Primordial egg (oogonium) of new-born cat, dividmg. C. Egg of 

 new-born cat containing attraction-sphere (5), and centrosome. D. Of young thrush surrounded 

 by follicle and containing besides the nucleus an attraction-sphere and centrosome (j), and a 

 yolk-nucleus (>'.«.). E. Of young chick containing nucleus, attraction-sphere and fatty deuto- 

 plasm-spheres (black). F. Egg of new-born child, surrounded by follicle and containing nucleus 

 and attraction-sphere. 



those of myriapods, having the form of a single well-defined sphe- 

 roidal mass which appears at a very early period and persists through- 

 out the later ovarian history. At the other extreme are "diffused 

 yolk-nuclei " having the form of numerous irregular and ill-defined 

 masses scattered through the cytoplasm, as described by Stuhlmann 

 ('86) in the eggs of insects and more recently by Calkins and Foot in 

 earthworms. An intermediate form is represented in the amphibia 



I 



