140 G. CARL HTJBER 



not to be explained on the supposition of superfecundation or 

 superfoetation. The record for this rat does not show insemi- 

 nation on successive days. At The Wistar Institute, after all 

 of the supposedly successful matings of albino rats, the females 

 rats are caged apart from the males. The smaller egg-cylinder, 

 though appreciably smaller, is in stage of development separated 

 from the other by a time interval of perhaps less than 24 hours. 

 It presents a stage of development which is comparable to C 

 of figure 27 (8 days) and except for size, to the one figured in 

 figure 29 (8 days, 17 hours) of Part I. It is believed that in this 

 case both ova were seminated at about the same time, and pro- 

 ceeded through normal segmentation and that on reaching the 

 lumen of the uterus during the fifth day they became lodged in 

 close proximity in the same mucosal fold. With the development 

 of the decidual crypts, both became enclosed within the same 

 crypt, at perhaps slightly different levels. In further develop- 

 ment one blastodermic vesicle dominated the other and from 

 about the seventh day on, one developed and differentiated 

 more rapidly than the other. Had development continued, two 

 distinct embryos, with separate amniotic cavities, attached to 

 the same placenta, would have been formed, with one embryo 

 large and more fully developed than the other. From mere 

 difference in size and of development of embryos in the same 

 litter it is not warranted to postulate superfecundation nor super- 

 foetation. I am of the opinion that usually when two morula 

 masses are lodged in close proximity in the same mucosal fold, 

 one or the other degenerates (fig. 2, A) and that the normal 

 development of both, as in the preparation shown in figure 10, 

 is of very rare occurrence. 



CONCLUSIONS 



A study of the abnormal or pathologic ova met with in the ex- 

 tended series of preparations covering the first ten days of the 

 development of the albino rat, enables grouping them in two 

 main classes: 



a. Such in which all of the ova of a given rat show, or are 

 associated with, abnormal development. 



