THE POLE DISC OF CHRYSOMELID EGGS. 



183 



A series of divisions now ensues until a number of bodies, like 

 chromosomes in appearance, is produced in a single space (Fig. 

 4, c). After about four or five divisions have taken place, they 

 begin to lose their affinity for the basic stain and also their regu- 

 larity of outline (Fig. 4, d), and in 

 the next stage (Fig. 4, e) they are 

 seen to have fragmented into nu- 

 merous irregular bodies that stain 

 with the acid dye. Still later (Fig. 

 4, /") the space is practically filled 

 with a finely granular acid staining 

 yolk mass. 



The process of yolk formation, 

 therefore, consists in the transfor- 

 mation of the basic staining granules 

 of the food stream into an acid stain- 

 ing yolk which fills the meshes of 

 the cytoplasmic reticulum of the 

 mature egg. In this transforma- 

 tion the cytoplasm as well as the 

 nucleus is undoubtedly involved. 

 The yolk is first laid down in the center of the egg, but as the 

 granules spread outward as well as inward from the nutritive 

 stream it is not long before yolk is found on either side of the food 

 stream. 



The pole disc does not appear until the egg is nearly mature, 

 and a study of successive stages shows that it is composed of 

 granules of the food stream that have accumulated at the pos- 

 terior end of the egg (Fig. 5). These granules are much 

 larger than those of the Keimhaut (the peripheral layer of cyto- 

 plasm surrounding the yolk in the mature egg) and stain in- 

 tensely with the basic stain. This staining reaction might sug- 

 gest that the granules are of nuclear origin ; but while the 

 nucleus at this time is far from inactive, with an interchange of 

 materials between it and the cytoplasm probably taking place, I 

 have never been able to observe an actual emigration of granules 

 from the nucleus into the cytoplasm. 



Union of the germ nuclei in fertilization and the early cleav- 



Fig. 3. Section of the cytoplasm 

 of ilie growing ovocyte in the region 

 of the nutritive stream. X 2,000. 



