THE DEVELOPMENT OP THE MOLE. 205 



with regard to the early phases of development, throw some 

 light upon the curious differences which then occur. 



Kolliker (No. 14, 2nd edit., p. 44) and Schulin (No. 30), 

 declare that the human ovum is markedly deficient in yolk 

 vesicles when compared with the ovum of the Cat or the Cow. 

 Bischoff (Nos. 8, 9, 10, 11), in his figures of the ova of 

 the Rabbit, Dog, Guinea-Pig, and Deer, shows that the 

 Deer^s ovum is not filled with such a dense mass of yolk as is 

 that either of the Dog or Rabbit, while the ovum of the 

 Guinea-Pig is remarkably transparent, a statement in the 

 latter case with which Reichert's (No. 18) and my own obser- 

 vations fully coincide {vide fig. 21). The Mole's ovum must 

 be classed in this particular with that of the Rabbit and Dog, 

 while the Bat's ovum, it appears, is similar to that of the 

 Guinea-Pig. 



The network, which has as far as I know hitherto only been 

 observed in Mammalian ova by Schafer (No. 19) in young 

 ovarian ova of the Rabbit, was very distinct in the ovum repre- 

 sented in fig. 2. A similar appearance was noted in other 

 ova, but in a considerable number no such network could be 

 detected. There appears to me, however, good reason to 

 believe that the appearance is due to a protoplasmic reticulum 

 in the meshes of which the food material lies. 



The Nucleus. 



In all those ova in which the nucleus was observed it was 

 placed excentrically, the density of the yolk being so great 

 it could not be distinguished when lying in the centre of the 

 ovum. It was found to be either circular or oval in optical 

 section, and bounded by a distinct membrane. In the ovum 

 represented in fig. 2 the nucleus is indicated by a circular ring ; 

 its contents could not, however, be observed owing to the density 

 of the supervening yolk, the network before spoken of being 

 seen overlying the nucleus. 



In figs. 3, 4, 5, I have drawn the nuclei of three ova which 

 I obtained from the female from which the ovum drawn in 



