EMBRYOLOGY OF CRYPTOBRANCHUS 101 



eggs may have been slow in undergoing maturation changes, or 

 defective in some way. 



In the great majority of cases of females taken with all the eggs 

 in the uteri, artificial fertilization has been successfully performed; 

 a high percentage of fertilized eggs is reached when all the eggs 

 show a distinct pit at the animal pole. In every case in which 

 seminal fluid was examined under the microscope during the breed- 

 ing season, the spermatozoa were motile; so it is not likely that 

 any cases of failure in artificial fertilization were due to defective 

 spermatozoa. 



The evidence indicates that the eggs are incapable of fertili- 

 zation at the time when the first eggs reach the uterus, but that 

 about the time all the eggs reach the uterus the majority of them 

 become capable of fertilization. This change in their potentiality 

 coincides in time with, or slightly precedes, the formation of a 

 distinct pit at the animal pole; it is probably correlated with the 

 formation of the second polar spindle (see section V). 



C. CHANGES VISIBLE FROM THE SURFACE DURING FERTILIZATION 



The appearance of the blastodisc shortly after fertilization is 

 shown in figs. 7 and 8. During the first eight hours after fertili- 

 zation there is an increase in the extent of the blastodisc from a 

 diameter of 90° to 130°-160°, with a corresponding increase in 

 the intensity of its differentiation. From this time up to first 

 cleavage there is no constant increase in the extent of the blastq- 

 disc, though the transition from the blastodisc to the darker region 

 surrounding the vegetal pole becomes more gradual. The pit 

 at the animal pole persists unchanged almost up to the time of 

 first cleavage; it is sometimes double (see fig. 10). Shortly before 

 first cleavage it becomes broader and shallower, and usually dis- 

 appears before the beginning of the first cleavage furrow. 



As early as fifteen minutes after artificial fertilization, pits or 

 scars made by the actual or attempted entrance of a spermatozoon 

 have been found on the surface of the egg. It seems remarkable 

 that the spermatozoon can pierce through the thick and tough 

 gelatinous capsule in so short a time. In living material, the 



