IMPORTANCE OF THE SPERM ASTER 259 



by some physiological condition inherent in the organization of the egg. In 

 the majority of chordate eggs, the place of sperm penetration is at that pole 

 of the egg which contains the egg nuclear material, although in some, such 

 as in the gobiid fish (fig. 123), the micropyle, permitting the sperm to get 

 through the egg membrane, may be situated at a point opposite the nuclear 

 pole of the egg. 



G. Monospermic and Polyspermic Eggs 



In the eggs of most animal species only one sperm normally enters the egg. 

 Such eggs are known as monospermic eggs. Among the chordates, the eggs 

 of Styela, Amphioxus, frog, toad, and mammals are monospermic. Abnormal 

 cleavage and early death of the embryo is the general result of dispermy and 

 polyspermy in frogs (Brachet, A., '12; Herlant, '11). In those chordates 

 whose eggs possess much yolk, the eggs are normally polyspermic, and several 

 sperm enter the egg at fertilization, although only one male pronucleus enters 

 into syngamic relationship with the egg pronucleus; the other sperm soon de- 

 generate and die in most cases (fig. 138). (See Fankhauser, '48.) In some 

 urodele amphibia, it appears that syngamic conjugation of more than one 

 sperm pronucleus with the egg pronucleus may occur in certain instances and 

 may give origin to heteroplcidy, and development may be quite normal 

 (Fankhauser, '45). Examples of normal polyspermic eggs are: birds, reptiles, 

 tailed amphibia, elasmobranch fishes, and possibly some teleost fishes. Among 

 the invertebrates, polyspermy is found in some insects and in the Bryozoa. 

 In the sea urchin, polyspermy may occur, but abnormal embryos are the rule 

 in such cases as indicated above. Similar conditions are found in certain other 

 invertebrates (Morgan, '27, pp. 84-86). 



Two explanations of normal polyspermy are suggested: 



( 1 ) The presence of a superabundance of yolk hinders the operation of 

 the mechanism whereby the egg inhibits the entrance of extra sperm; 

 the egg, therefore, falls back upon a second line of defense within its 

 own substance which excludes the sperm from taking part in or hin- 

 dering the normal functioning of the syngamic nucleus in its relation 

 to development; and 



(2) a large amount of yolk makes it advantageous to the egg for extra 

 sperm to enter, as they may contribute enzymes or other substances 

 which enable the egg better to carry on the metabolism necessary in 

 utilizing yolk material. 



H. Importance of the Sperm Aster and the Origin of the First 

 Cleavage Amphiaster 



One of the older views of fertilization maintained that the egg possessed 

 the cytoplasm but lacked a potent centrosome or "cell center" capable of 



