104 BERTRAM G. SMITH 



sections it appears that the first three types of sperm pits indicate 

 that the spermatozoon has barely penetrated through the cell 

 wall, or that the attempt is an abortive one; the last two types 

 indicate with considerable certainty that the spermatozoon has 

 penetrated well into the egg. 



Polyspermy is the rule. Cases of penetration by more than one 

 spermatozoon have been found fifteen minutes after fertilization, 

 while the surface of the egg may be scarred by a dozen or more 

 wounds presumably made by other spermatozoa. An hour later, 

 the majority of the eggs have been penetrated each by from one 

 to ten spermatozoa, and sometimes scarred by as many as fifty 

 more. In one case observed the entire number of sperm pits 

 reached nearly a hundred. 



About three hours after fertilization the small white spot 

 representing the sperm pit is surrounded by a circular area about 

 10° to 15° in diameter, slightly darker than the general surface 

 (see fig. 9) . An hour later this area has increased in size, is whiter 

 throughout its central portion, and is sharply bounded by a dark 

 line which forms a perfect circle (see fig. 10). This dark line is, 

 partly at least, due to a slight depression in the general surface of 

 ^ the egg. For convenience the area enclosed by this circle will be 

 called the 'sperm area.' 



During the next few hours the sperm area increases in size 

 until it covers almost an entire hemisphere (figs. 11 and 12). Its 

 surface is now in general a trifle paler than the remainder of the 

 egg outside the blastodisc ; its boundary may pass the animal pole 

 without interruption. Two or even three sperm areas in this 

 advanced stage may be present, their boundaries usually over- 

 lapping. Fig. 12 shows an egg fertilized from two opposite sides, 

 the spermatozoa entering near the margin of the blastodisc; 

 the two sperm areas meet at the animal pole, but remain widely 

 separated in the lower hemisphere. 



In monospermic eggs preserved and dissected in this stage, 

 the sperm area is found to overlie a lenticular or disc-shaped 

 mass, of firmer consistency than the remainder of the egg which 

 may sometimes be shelled off in a few concentric layers like the 

 fleshy part of an onion. Outside the boundaries of both sperm 



