22 CARL GOTTFRIED HARTMAN 



obvious polarity is to be recognized in several eggs of batch No. 

 52 in which no chromatin is seen, but in other eggs of the same 

 batch with similar absence of well-stained chromatic substance, 

 the yolk is peripherally situated. Whether one study a given 

 section through the center of the egg or the series of sections as 

 a whole, no striking condition of polar differentiation into yolk 

 mass and cytoplasm is encountered nor are minor polar differ- 

 ences in structure of the egg at all of uniform occurrence during 

 the pronuclear stage. 



From these facts, it is apparent that the pronuclei, at first 

 eccentrically situated, soon migrate to the center, and the yolk 

 maintains its peripheral distribution. This interpretation finds 

 ample confirmation from a study of the next stage. 



THE FIRST CLEAVAGE 



a. The first cleavage spindle 



The first cleavage spindle was observed in two specimens: 

 No. 54 (4) b in metaphase (fig. 7) and No. 52 (3) a in early 

 anaphase stage of division (fig. 8). The former shows a slight 

 polarity in the distribution of yolk vacuoles, but the chromosomes 

 are situated centrally. In the other specimen, there is no trace 

 of polarity. The central area is perfectly yolk-free and is largely 

 occupied by the spindle, only partly shown in figure 8. At the 

 periphery there ig a considerable number of fragments of yolk 

 eliminated (not shown in the figure). At one side a new cell 

 membrane appears to be forming just within the eliminated yolk- 

 laden fragments. 



The formation of the first cleavage spindle from the pronuclei 

 was hot observed, but sufficient material has been described to 

 make it certain that the entire egg in the first cleavage divides 

 approximately, if not exactly, into equal halves and that the 

 yolk is not cast off in one large polar mass as in Dasyurus. That 

 the yolk is given off at the entire periphery is shown by the: 

 stage just described and by the two-celled stage to be discussed 

 in the next section. 



