THE IMMATURE EGG AND ITS MATURATION 73 



showing the presence of nucleic acid compounds. Casperson and 

 Schultz (1940) think these are of the ribose type (in Psammechinus mili- 

 aris). The appearance of the immature Arbacia egg with other stains is 

 described and illustrated by E. B. Wilson (1926) ; Benda-osmic, Benda- 

 Kull, Champy-Kull, Champy-osmic, thionin, toluidin blue have been 

 used. In some of these figures, bodies which appear to be nucleolini 

 inside the nucleolus show up clearly but are not mentioned in the 

 text. Some cytochemical techniques have been described by Krugelis 

 (1947 b): Gomori, fast green, toluidin blue, and Feulgen. 



With certain intravitam stains, the immature egg stains more inten- 

 sely than the fertilized t^g, and this more intensely than the unfer- 

 tilized (Lyon and Shackell, 1910b; E. N. Harvey, 1910c). 



The immature egg is surrounded, like the mature egg, by a layer of 

 jelly. In the older oocytes it is the same in amount as in the mature 

 egg and may be as much as 32 (jl thick. It is thinner in younger oocytes ; 

 in an oocyte whose diameter was 60 \i, the jelly was only 3.2 [x. The 

 jelly coat is discussed under the mature egg and further data may be 

 found in Part IV, Jelly Layer. 



c. Reaction to Sperm 



One of the most characteristic features of the immature egg is its reac- 

 tion to sperm. The formation of papillae or blebs (Photograph 16) 

 wherever sperm hit the surface is a well-known phenomenon in many 

 echinoderms including Asterias and holothurians (Hobson, 1927). 

 These papillae have many different forms; they may be blunt or 

 sharply pointed and are sometimes branching; they are always hyaline, 

 without granules. They have been carefully studied by Seifriz (1926) in 

 the sand dollar. The sperm enter the immature Arbacia tgg as (Photo- 

 graph 17) they do other sea urchin eggs, e.g., Lytechinus as described 

 by Wilson (1895) in his Atlas. But no fertilization membrane is raised. 

 The papillae are considered similar to fertilization cones which form 

 on mature eggs after fertilization. They are formed within two minutes 

 after adding sperm, and they last for about 20 minutes and are gra- 

 dually resorbed. They are formed on eggs with germinal vesicle intact, 

 after the germinal vesicle has broken, during polar body formation 

 and even an hour after the second polar body has been given off. They 

 do not form in Ca-free sea water though the sperm are motile; this 

 has been found also for Anthocidaris crassispina by Sugiyama (i938g) 

 and for Psammechinus miliaris by Rothschild and Swann (1949). It is 

 of interest that fertilization of mature eggs will not take place in Ca- 



