414 WALTER HEAPE. 



The zona is radially striated, and its outer edge has a 

 granular appearance, which I have reason to believe, from an 

 examination of ovarian ova, is due to the irregularity of its 

 surface caused by the pressure of the follicular epithelium upon 

 it while still in the ovary. 



There is no albumen deposited round the ovum during its 

 passage down the Fallopian tube, as is the case with the 

 rabbit^s ovum. 



The ovum within the zona measures '15 by 'ITmm., while 

 the inner mass measures "1 by '12 mm. in diameter ; the outer 

 layer being about '05 mm. thick. The zona is '01 mm. and 

 the outer coat "014 mm. in thickness. 



The size of segmenting ova vary somewhat, but as a rule, 

 while in the Fallopian tube, they measure between *08 and •! 

 mm. in diameter. This fully-segmented ovum shows a con- 

 siderable increase on that size, and this is probably due to the 

 absorption by it of nutritive material present in the uterus. 

 It was examined first of all while fresh, but the details of its 

 structure were rendered more apparent by treatment with silver 

 nitrate. The figure was drawn after treatment. 



The structure of the fully-segmented mole's ovum as de- 

 scribed above is identical with that of the fully-segmented 

 ovum of the rabbit which van Beneden has described (Nos. 4 

 and 5), According to this author the result of the first divi- 

 sion of the ovum of the rabbit is the formation of two cells, 

 the one of which is smaller and more granular than the other. 

 The product of these two cells can be distinguished from one 

 another throughout the process of segmentation, and Beneden 

 finds the cells derived from the granular segment become invo- 

 luted within those derived from the larger hyaline segment, 

 and two layers are thus formed which he terms "entoderm'^ 

 and " ectoderm" respectively, according to what he considers 

 is their respective fate. 



Further, the point where the involution took place remains 

 open in the fully-segmented ovum, and gives rise to the gap 

 in the outer layer, which is called by Beneden the " blasto- 



