X.] SEGMENTATION. 313 



exposed spot may for convenience be called with v. Bene- 

 den the blastopore, though, as will be seen by the ac- 

 count given of the subsequent development, it in no 

 way corresponds with the blastopore of other vertebrate 

 ova. 



In the following account of the segmentation of the rabbit's 

 ovum, v. Beneden's description is followed as far as the details 

 are concerned, his nomenclature is however not adhered to 1 . 



According to v. Beneden 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 outer spheres, and the smaller one 

 and its products as the inner 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. At the moment of their first separation these 

 spheres are spherical, and arranged in two layers, one of them 

 formed of the four outer, and the other of the four inner spheres. 

 This position is not long retained, for one of the inner spheres 

 passes to the centre ; and the whole ovum again takes a spherical 

 form. 



In the next phase of segmentation each of the four outer 

 spheres divides into two, and the ovum thus becomes constituted 

 of twelve spheres, eight outer and four inner. The outer spheres 

 have now become markedly smaller than the inner. 



The four inner spheres next divide giving rise, together with 

 the eight outer spheres, to sixteen spheres in all ; which are 

 nearly uniform in size. Of the eight inner spheres four soon 

 pass to the centre, while the eight now superficial outer spheres 

 form a kind of cup partially enclosing the inner spheres. The 

 outer spheres now divide in their turn, giving rise to sixteen 



1 The cells spoken of as the outer layer correspond to Van Beneden's 

 epiblast, whilst those cells spoken of as the inner correspond to his 

 primitive hypoblast. 



