CHAPTER X. 



MAMMALIA. 



THE classical researches of Bischoff on the embryology of 

 several mammalian types, as well as those of other observers, 

 have made us acquainted with the general form of the embryos of 

 the Placentalia, and have shewn that, except in the earliest stages 

 of development, there is a close agreement between them. More 

 recently Hensen, Schafer, Kolliker, Van Beneden and Lieber- 

 kiihn have shed a large amount of light on the obscurer points of 

 the earliest developmental periods, especially in the rabbit. For 

 the early stages the rabbit necessarily serves as type; but there 

 are grounds for thinking that not inconsiderable variations are 

 likely to be met with in other species, and it is not at present 

 easy to assign to some of the developmental features their true 

 value. We have no knowledge of the early development of the 

 Ornithodelphia or Marsupialia. 



The ovum on leaving the ovary is received by the fimbriated 

 extremity of the Fallopian tube, down which it slowly travels. 

 It is still invested by the zona radiata, and in the rabbit an al- 

 buminous envelope is formed around it in its passage downwards. 

 Impregnation takes place in the upper part of the Fallopian 

 tube, and is shortly followed by the segmentation, which is re- 

 markable amongst the Amniota for being complete. 



Although this process (the details of which have been made 

 known by the brilliant researches of Ed. van Beneden) has 

 already been shortly dealt with as it occurs in the rabbit (Vol. II. 

 p. 98) it will be convenient to describe it again with somewhat 

 greater detail. 



The ovum first divides into two nearly equal spheres, of 

 which one is slightly larger and more transparent than the 



