THE IMMATURE EGG AND ITS MATURATION 75 



f. Permeability to Water 



According to Churney (1941b, 1942), the permeability to water of 

 the immature egg is the same as the mature egg; the germinal vesicle 

 acts as a perfect osmometer, and the nucleolus swells and shrinks 

 reversibly. 



g. Polar Body Formation. Plate II 



After the immature egg has reached its full size, it throws off two polar 

 bodies. The first step in this process is the approach of the germinal 

 vesicle to the cell wall (Photograph 7). Then after a period of about 

 I J hours, during which the nucleolus has become smaller and dis- 

 appeared, the wall of the germinal vesicle breaks down; the red pig- 

 ment granules become more numerous and very bright in color in 

 this region, just inside the cell wall near the breaking germinal vesicle 

 (Photographs 7 and 8). The mitochondrial granules do not take part, 

 but are heaped up on the opposite side of the breaking germinal 

 vesicle, as can be seen in eggs stained with methyl green. A clear area 

 is seen where the germinal vesicle broke down, and two or two and a 

 half hours (23° C.) after the breakdown, the first polar body is given 

 off. One to one and a half hours later the second polar body is given 

 off. Then about two hours later, the mature nucleus is formed. During 

 the formation of the polar bodies the egg becomes somewhat flattened 

 in the axis of the polar bodies (Photograph 12). This lasts for about 

 7 minutes; then the egg becomes spherical again. 



By watching individual eggs fertihzed at intervals (23° C.) after the 

 second polar body has been given off, the following data have been 

 obtained: 



I to I hr. No fertilization membrane, delayed and irregular 

 cleavage, polyspermy, 3 cells at once, 

 I J to 1 1 hrs. Fertilization membrane, polyspermy, irregular cleav- 

 age, normal swimming blastulae. 



2 to 2 1 hrs. Cleavage delayed and slightly irregular, polyspermy, 



4 cells at once. 



3 to 5 hrs. Slight delay in first cleavage; cleavages regular, mi- 



cromeres normal, normal plutei. 



There is apparently a sort of cytoplasmic maturation following 

 nuclear maturation such as has been described for Paracentrotus lividus 

 by Paspaleff (1927) and for Psammechinus miliaris by Runnstrom and 

 Monne (1945) and Runnstrom (1948 b). The eggs do not develop 



