EMBRYOLOGY OF CRYPTOBRANCHUS 



133 



effect on the cell wall of penetration by the spermatozoon, de- 

 scribed later). The presence of the contact disc is uniformly 

 accompanied by a deficiency of the spindle, which lacks an aster 

 at the end nearest the disc. In a few cases there seems to be a 

 small amount of sphere substance underlying the contact disc. 

 The inference seems to be that the contact disc is the product of 

 the aster of the first polar spindle modified by contact with the 



c.d. 



mmm. 





"c^PJUi?^ 





•■. ' --:?-,-' ' ,vy>g! g 



mm^mm mmm 



o 



mm 



wO' 



iM 



Oo--,>:-r '■■■njiy'oOhCJp 







32 



33 



34 



35 



Figs. 32-35 Meridional sections showing first polar spindle of Cryptobranchus 

 allegheniensis. Figs. 32 to 34 are from ovarian eggs; fig. 35 is from an egg taken 

 from the lower part of the oviduct. X 340. 



Fig. 32 c. w., cell wall, formed from the zona radiata. z. p., zona pellucida. 



Fig. 33 c. d., contact disc. 



Fig. 34 The section cuts the spindle obliquely and includes all the chromatin 

 except one small chromosome belonging to the central part of the group, which is 

 left in an adjacent section. There are probably six large chromosomes forming a 

 ring, surrounding six small chromosomes in a state of division. 



Fig. 35 A considerable part of the chromatin is left in an adjacent section. 

 There are probably six large and six small chromosomes, arranged ipuch as in the 

 preceding figure. 



cell wall. The function of the disc, if it have any function, may 

 be to anchor the spindle at the surface during the pulling-apart 

 of the two sets of chromosomes. Unfortunately for this hypothe- 

 sis the linin threads have not been traced from the chromosomes 

 of the first polar spindle to the contact disc; but since the latter 

 structure is never found except in conjunction with a polar spindle, 

 there is no escape from the conclusion that it is in some way related 

 to it. 



