TWINNING IN DASYPUS NOVEMCINCTUS 35 



toplasmic zones. This pronounced shifting about of 

 materials within the maturing ovocyte must take place 

 in a very brief space of time, for it has not been possible 

 to find in a very large number of specimens examined any 

 stages transitional between that before the reorganiza- 

 tion begins (Fig. 5) and that after its completion (Fig. 6). 



In attempting to interpret the significance of this 

 remarkable cellular cataclysm it is interesting to note 

 that Hill' finds exactly the same situation in the marsu- 

 pial Dasyurus, which he interprets as a complete reversal 

 of polarity. According to this writer the formative 

 protoplasm now occupies the vegetative pole while the 

 yolk mass occupies the animal pole. The position of 

 the maturation spindle is in the formative zone, but 

 as near the animal pole as possible. Whatever may be 

 the morphological interpretation of the changes in 

 cellular organization that take place during maturation, 

 there can be httle doubt as to the physiological sig- 

 nificance of the shifts of material. If we may judge 

 by analogy with Dasyurus and by a study of partheno- 

 genetic cleavage in atretic follicles of the armadillo, 

 the shift of the deutoplasm from the center to the 

 periphery of the ovocyte is the first step in the process 

 of deutoplasmic extrusion. This mass of thin degenerate 

 yolk is of no value to the egg and must be voided before 

 cleavage can begin. The process of voidance appears 

 to be one involving rupture of the vitelline membrane 

 and abstriction of the yolk, followed by a subsequent 

 rounding up of the formative materials to form an egg 

 that is much smaller than the original ovocyte and 

 is presumably rejuvenated by the loss of its deutoplasm. 



' J. P. Hill, Quarterly Journal of Microscopical Science, LVI (1910). 



