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, KoUiker, Van Beneden and Lieberkiihn have shed a 

 large amount of light on the obscurer points of the earliest deve- 

 lopmental 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 Mar- 

 supialia. 



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 albuminous 

 envelope is formed around it in its passage downwards. Impreg- 

 nation takes place in the upper part of the Fallopian tube, and is 

 shortly followed by the segmentation, which is remarkable 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. I. p. 80) 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 other. The 

 larger sphere and its products will be spoken of as the epiblastic 

 spheres, and the smaller one and its products as the hypoblastic 

 spheres, in accordance with their different destinations. 



Both the spheres are soon divided into two, and each of the four 

 so formed into two again; and thus a stage with eight spheres ensues. 



B. E. II. 12 



