326 EDWIN G. CONKLIN 



found in the fact that in the former fertihzation does not take 

 place until after the first maturation division is completed and 

 then each of the daughter cells is fertilized, whereas in the latter 

 the entrance of the spermatozoon occurs before thfe completion 

 of the first maturation division, with the result that one of the 

 daughter cells contains a spermatozoon and the other does not. 



In Crepidula the spermatozoon usually enters the egg at the 

 time when the germinal vesicle dissolves and always before the 

 first polar body is cut off. In many mollusks, annelids and ascid- 

 ians, the first maturation spindle remains in the metaphase 

 until the spermatozoon enters the egg or until the egg is stimu- 

 lated by other means (artificial parthenogenesis) to begin de- 

 velopment. The giant polar bodies of Crepidula behave like 

 unfertilized eggs in these regards: 1) the chromosomes do not 

 usually unite to form a daughter nucleus but remain as if they 

 were in the metaphase, as in Chaetopterus, Ciona, etc., though 

 no distinct spindle is visible (figs. 38, 40 to 46). 2) They also 

 resemble unfertilized eggs in that the whole cell stains a purple 

 color in picro-haematoxylin showing that cytoplasm is diffused 

 throughout the whole cell, whereas after fertilization there is a 

 fairly sharp separation of cytoplasm and yolk, the former alone 

 staining purple. 3) Associated with this lack of segregation of 

 cell substances in giant polar bodies there is a lack of the move- 

 ments which in the fertilized egg lead to the segregation of cyto- 

 plasm at the animal pole and of yolk at the vegetal one. 



These giant polar bodies contain samples of all the ooplasmic 

 substances; they may be larger than the ootid which does develop, 

 but the one thing which they lack is a spermatozoon, whereas 

 that ootid which does develop invariably contains a spermato- 

 zoon; we must conclude therefore that the giant polar bodies of 

 Crepidula do not develop because they are not fertilized, and 

 they are not fertilized because a spermatozoon had entered the 

 egg before their formation, thus rendering the polar bodies as 

 well as the egg impervious to other spermatozoa. 



In this fact is to be found the explanation of the different 

 behavior of the giant polar bodies of Prostheceraeus and of other 

 animals, e.g., Crepidula, for it is well known that one of the first 



