THE DEVELOPMENT OF THE MOLE. by 
one spot where the polar bodies are formed. At this latter 
place two polar bodies may be seen in the specimen figured, 
outside the vitelline membrane, whilst the nucleus remains 
as the female pronucleus lying in the peripheral portion of the 
ovum. Finally, the vitellus again expands and the nucleus 
retires to the centre of the ovum and is no longer to be seen. 
Assuming that these observations are correct, Beneden’s descrip- 
tion of the ejection of the vesicle to form the polar bodies and 
the subsequent non-nucleated condition of the ovum must be 
considered erroneous. 
Impregnation appears to be effected by asingle spermatazoon, 
although a considerable number of spermatazoa find their way 
through the zona and may be seen lying passively in the circum- 
vitelline space. 
The segmentation occurs while the ovum travels down the 
Fallopian tube. Twoand then four segments are formed, after 
which the course of segmentation is irregular. The segments 
themselves are of irregular size and do not appear to be divi- 
sible into two kinds (epiblastic and hypoblastic) as Beneden 
describes. After its entrance into the uterus, a division of the 
segments into an outer hyaline layer and inner deeply granular 
mass takes place, and I would suggest the hypothesis that the 
vitelline matter which was originally contained in all segments 
alike has been transmitted from the outer segments to the 
segments lying in the interior of the ovum, in order that the 
former segments may the more readily and actively multiply 
and flatten out to form the wall of the blastodermic vesicle. 
The epiblast of the vesicle and of the embryo is derived from 
the whole of the outer layer and by far the largest proportion 
of the inner mass of segments. The hypoblast is derived from 
the small remaining portion of the inner mass and the meso- 
blast, subsequently, from both epiblast and hypoblast layers. 
This being the case, the division of the segmentation spheres, 
by Beneden, into epiblast and hypoblast spheres from the time 
when the first two segments were formed, is incorrect ; and at 
the same time the theory of a comparison of the metagastrula 
stage with the gastrula of other animals is likewise untenable, 
