EMBRYOLOGY OF CRYPTOBRANCHUS 145 



'copulation path' (see Hertwig, '06, p. 529) is not dependent 

 upon the rotation of the sperm head. 



(6). Polyspermy, and the fate of the supernumerary spermatozoa. 

 Brief data regarding the occurrence of polyspermy have already 

 been given. It is possible that the method of artificial fertili- 

 zation increases the number of spermatozoa entering the egg ; but 

 in nature the eggs are fertilized in a confined space, and I see no 

 reason to doubt that polyspermy is a common occurrence under 

 natural as well as artificial conditions. It is evident that we have 

 here to deal with physiological, not induced or accidental poly- 

 spermy (see Brachet, '10), for the eggs develop in a normal manner. 



While the distribution of spermatozoa entering the egg is largely 

 if not entirely a matter of chance, the location in which a sperma- 

 tozoon finds itself has much to do with its ultimate fate. Sper- 

 matozoa entering the lower hemisphere, especially in the region of 

 the vegetal pole, never penetrate far, and since they are found in 

 this hemisphere only during the first few hours after fertilization, 

 must quickly degenerate. In the urodele Hynobius, Kunitomo 

 ('10) found that a spermatozoon entering at the vegetal pole 

 sometimes succeeds in reaching the egg-nucleus; but the careful 

 study of many eggs has convinced me that this never occurs in the 

 heavily yolk-laden and strongly telolecithal egg of Cryptobranchus. 



Only in the blastodisc have spermatozoa been found in the stage 

 characterized by the presence of the cytoplasmic crescent (figs. 

 43 and 44). Obviously, the conditions elsewhere are unfavorable 

 for the formation of any considerable mass of cytoplasm about 

 the spermatozoon. In the stage with a well-formed cytoplasmic 

 crescent, not more than two spermatozoa have been found in a 

 single egg. No accessory spermatozoa have been found in any 

 situation after the formation of a sperm-nucleus. The supernu- 

 merary spermatozoa thus have but a transient existence, and the 

 only advantage resulting from polyspermy is doubtless that, 

 in an egg so large, penetration by several spermatozoa is of value 

 in insuring fertilization. 



The literature on the occurrence of polyspermy in the amphib- 

 ian egg has recently been reviewed by Kunitomo ('10). As 

 noted in a previous paper (Smith, '11), I have found polyspermy 



