MICROPYLES 257 



that several sperm enter the egg in the sea urchin if insemination is permitted 

 before the maturation divisions occur. The immature egg, therefore, lacks the 

 mechanism for the control of sperm entrance. Moreover, A. Brachet ('22) 

 and Bataillon ('29) demonstrated that the sperm nuclei and asters behave 

 abnormally under these conditions, and normal development is impossible. 

 Runnstrom and Monne have further shown for the sea-urchin egg that the 

 normal fertilization process, permitting the entrance of but one sperm, requires 

 a mechanism which is built up gradually by degrees during the time when 

 the maturation divisions of the egg occur, even extending to a necessary short 

 period after the divisions are completed. Not only is the mechanism which 

 permits but one sperm to enter the egg established at this time in the sea- 

 urchin egg, but Runnstrom and Monne further conclude, p. 25, "that the 

 cytoplasmic maturation" which occurs at the period of the maturation divi- 

 sions, "involves the accumulation at the egg surface of substances which 

 participate in the activating reactions." It appears, therefore, that the break- 

 down of the germinal vesicle together with the phenomena associated with the 

 maturation divisions is an all-important period of oocyte development, con- 

 trolling sperm entrance on the one hand and, on the other, presumably being 

 concerned with formation of substances which permit egg activation. 



F. Micropyles and Other Physiologically Determined Areas for 

 Sperm Entrance 



A micropyle is a specialized structural opening in the membrane or mem- 

 branes surrounding many eggs which permits the sperm to enter the egg. 

 For example, in the eggs of teleostean fishes or in the eggs of cyclostomatous 

 fishes, a small opening through the vitelline membrane (or chorion) at one 

 pole of the egg permits the sperm to enter (figs. 93 A; 134A-F). On the 

 other hand, many chordate eggs do not possess a specialized micropyle through 

 the egg membrane. The latter condition is found in the protochordates, Styela 

 and Amphioxus, and in vertebrates in general other than the fish group. In 

 Styela and Amphioxus the sperm enters the vegetal pole of the egg, i.e., the 

 pole opposite the animal or nuclear pole. In most of the vertebrate species 

 the sperm enters the animal or nuclear pole of the egg usually to one side of 

 the area where the maturation divisions occur (figs. 115, 118, 1 191). In urodele 

 amphibians, the passage of several sperm into the egg at the time of fertili- 

 zation complicates the picture. However, the sperm which finally conjugates 

 with the egg pronucleus is the one nearest the area where the egg pronucleus 

 is located. The several sperm entering other parts of the egg ultimately de- 

 generate (fig. 138). Presumably this condition is present in reptiles and birds 

 where many sperm normally enter the egg at fertilization. 



In conclusion, therefore, it may be stated that the point of sperm entrance 

 in chordate eggs in general appears to be definitely related to one area of 

 the egg, either by the presence of a morphologically developed micropyle or 



