OOGENESIS AND EARLY EMBRYOLOGY ASCARIS 563 



tion of the Querkerbe. These cells (fig. 66) exhibit a great 

 number of chromosomes, — between sixty-five and seventy-five, — 

 perhaps more correctly called karyosomes, which are almost spheri- 

 cal, and show no constriction in any plane. It seems probable 

 that these chromatic bodies result from the splitting or frag- 

 mentation of the chromosomes along the plane of the Querkerbe. 

 Thus the Querkerbe may indicate the plane of division of the 

 complex chromosomes of the sexual cells into their ultimate 

 components, such division into simple chromosomes occurring 

 only in cells about to undergo the process of 'diminution.' The 

 variation in the number of these ultimate chromosomes is prob- 

 ably due to the sex of the embryo, depending upon the number 

 of male chromosomes — twelve or eighteen — added to the eighteen 

 of the egg. 



In A. canis, as was shown in A. megalocephala by Boveri 

 ('88), the stem cell (figs. 68, 69, and 71) is ordinarily the smaller 

 of the two daughter cells of the first cleavage division. When 

 the two cells are in the resting condition (fig. 68), this criterion 

 is the only one available for distinguishing the soma {so.) from 

 the stem cell. After the nuclear rearrangements, the chromatin 

 of the two-cell stage is gathered into discrete chromosomes 

 (fig. 68) of the same number and form as those which enter 

 the daughter plates, that is there are either thirty or thirty-six 

 dyad chromosomes in each nucleus. 



As the second cleavage commences, a further difference be- 

 tween the two cells is manifested. In practically all cases, the 

 soma cell (figs. 69 to 72, so.) is the one which begins to divide 

 first. The axis of its spindle figure is parallel to the first cleav- 

 age plane; but the spindle of the stem cell, formed later, is per- 

 pendicular to that plane (fig. 73). 



It is a feature of all the cleavage cells, first manifested in the 

 two-cell stage, that the centrosome retains its identity from cell 

 generation to cell generation. Examination of the cells at any 

 time between mitoses shows the centrosome persisting as a dis- 

 crete, clearly distinguishable granule (figs. 72, 74 to 76) sur- 

 rounded by a small envelope of cytoplasm, which is slightly 

 more refractive than the rest of the cytoplasm. 



