96 BERTRAM G. SMITH 



IV. THE EXTERNAL HISTORY OF THE EGG BEFORE CLEAVAGE 



Some superficial aspects of the history of the egg before cleavage 

 have already been considered in connection with the account of 

 the breeding habits. 



A. EXTERNAL CHANGES PRECEDING AND ACCOMPANYING 



MATURATION 



Except where otherwise mentioned, the observations recorded 

 under this heading were made on the living egg. 



If the ovary of an adult Cryptobranchus be examined at any 

 time during the summer, the eggs which are about to become ma- 

 ture are readily distinguishable by their much greater size and 

 yolk content. 



In the living ovaries of adults taken about the middle of August, 

 the eggs show no positive surface indications of a telolecithal 

 structure. The same eggs fixed by a variety of methods show a 

 circular area or ' calotte' about 60° in diameter, which is somewhat 

 lighter in color than the remaining surface of the egg. On account 

 of its large size the egg now causes the ovarian wall to bulge strongly 

 outward. In general the pale circular area is situated in the cen- 

 ter of the more exposed hemisphere of the egg, and is not so pro- 

 fusely covered with ovarian blood-vessels as the remainder of this 

 hemisphere, but this relation is not always exact. 



Sections show that the calotte is the outward expression of a 

 peripheral disc-shaped region richer in protoplasm and small 

 yolk granules than the remainder of the egg; in the center of this 

 disc lies the germinal vesicle. From its homologue in the teleos- 

 tean egg I shall call this region the germinal disc or blastodisc; 

 in surface views it may be referred to by the same names, or more 

 strictly speaking, as the germinal area. Fixation serves to accen- 

 tuate the optical differences between the germinal disc and the 

 remainder of the egg, making the germinal area visible in pre- 

 served material at an earlier stage than in the living egg. The 

 center of the germinal area defines the animal pole of the egg. 



Shortly before the egg is ready to leave the ovary, the germinal 

 vesicle appears at the very surface, at the center of the germinal 



