ORGAN-FORMING SUBSTANCES IN EGGS OF ASCIUIANS. 2OQ 



and certainly nothing like this has ever before been photographed 

 in the living condition. I wish here to express my great indebt- 

 edness to Misses Foot and Strobell for their kindness in making 

 the photographs and also in superintending the production of 

 the plate. 



DIFFERENTIATIONS AND LOCALIZATIONS OF THE OOPLASM. 



In the ovarian egg of Cynthia there is a peripheral layer of 

 protoplasm, free from yolk, in which the "test cells" are im- 

 bedded and which contains numerous orange-yellow pigment 

 granules which are uniformly distributed. The central portion 

 of the egg consists of yolk-laden protoplasm which is slate-gray 

 in color and lying somewhat eccentrically within the egg is the 

 large, clear germinal vesicle. Before the egg leaves the ovary 

 the chorion is formed and the "test cells" are extruded into the 

 space between the egg and the chorion. 



After the egg is laid the germinal vesicle breaks down, but the 

 polar bodies are not formed until after the egg is fertilized. If 

 fertilization does not take place the polar bodies are never 

 extruded, and the clear, the yellow and the gray substances re- 

 main in the positions in which they were before the wall of the 

 germinal vesicle disappears. If the egg is fertilized, however, a 

 most astonishing series of movements occur which lead to the 

 localization of the different ooplasmic substances in definite 

 regions of the egg. 



The spermatozoon always enters the egg near the vegetal pole 

 and immediately after its entrance the peripheral layer of yellow 

 protoplasm flows rapidly to this pole where it collects as a cap l 

 (Photos i and 2). At the same time the clear substance derived 

 from the germinal vesicle also flows to the lower pole where it 

 lies between the cap of yellow protoplasm below and the yolk 

 above. This streaming of protoplasm to the lower pole takes 

 place so rapidly that its movements can be directly observed. 

 Within ten or fifteen minutes after the entrance of the sperm into 

 the egg all of the clear and yellow protoplasm has collected at 

 the lower pole while the opposite pole where the polar bodies 



1 In all the photographs the yellow substance appears very dark, the gray sub- 

 stance less dark while the clear protoplasm is relatively light. 



