THE DEVELOPMENT OF THE MOLE. 215 



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 vitelliis 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 a single spermatozoon, 

 although a considerable number of spermatozoa 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. Two and 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. 



