OOGENESIS AND EARLY EMBRYOLOGY ASCARIS 577 



spindle formation by the two pronuclei without their first fusing 

 to form one nucleus. The membranes of the two pronuclei, 

 which lie side by side somewhat eccentrically and nearer the 

 animal pole of the egg, disappear, not only on the adjacent faces 

 of the nuclei, but also in the regions nearest the centrosomes. 

 Thus the chromosomes of the two origins come to lie in different 

 portions of the cleavage spindle. Whether this separation is a 

 permanent one, as is the case in A. megalocephala, is a question 

 not as yet answered, for there is not sufficient morphological 

 difference between the chromosomes of male and female origin 

 in A. canis to allow of their identification when together in the 

 same spindle. The process of fertilization is completed during 

 the reorganization of the nuclei of the two daughter cells of the 

 first cleavage division. 



Apparently the first two cleavage cells are exactly alike, save 

 for a slight difference in size, and the difference between the soma 

 and the stem cell is seldom recognizable until the second cleav- 

 age division, and often not until the third. This difference lies in 

 the fact that ' diminution' takes place in the soma cell at its first di- 

 vision after separating from the stem cell. As shown by Zoja ('96) 

 in A. megalocephala, this 'diminution' process may be, and in A. 

 canis very commonly is, deferred until the division of the cells de- 

 rived from the original soma cell. Thus two ' diminution' figures 

 appear when, in the four-cell stage two soma cells divide to produce 

 the six-cell stage. There is one rather unreliable means of distin- 

 guishing between the first stem cell and the first soma cell even when 

 both divide by ordinary mitosis (that is, without ' diminution' in 

 either cell) , for the soma cell is ordinarily the first to commence 

 division. 



The ordinary 'T-shaped' embryo of the four-cell stage is 

 formed, with the propagation cell at the free end of the up- 

 right of the 'T'. This cell rotates upon the second soma cell 

 (also in the upright of the 'T') until the embryo assumes a 

 lozenge-shape with the stem cell at one of the acute angles. In 

 the cases where the 'diminution' process has been delayed, the 

 division from the four- into the six-cell stage shows two ' diminu- 

 tion' spindles. This process of 'diminution' takes place in each 



JOURMAL OF MORPHOLOGY, VOL. 30, NO. 2 



