564 



SEGMENTATION IN RABBIT 



ciUiatLil iiitu iui outer ccludcrm and an inner, lowci layer uf cells 

 (compare Fig. 119), between which and the yolk the enteric cavity is 

 formed : a segmentation-cavity is hardly recognisiblc. As the embryo 

 develops, it becomes folded off from the yolk, which forms -.x yolk-sac on 

 its ventral side (Figs, 120 and 154). 



The minute egg of the rabbit and of most other Mammals, although 

 alccithal and undergoing a holoblastic segmentation, has presumably 



t IG. 146.— OcspLTln of i.ilDbit 70-90 lioiirs aftci iniprcgiialion. 

 /f.c.iMty of blastodermic \csicle (yolk-sac) ; <■/• "uler layer of cells (Irophoblasl) ; 

 Aj'. inner mass of cells of Ihe embryonic area ; ;'/. albuminous envelope. (From 

 Lalfour, aUcr E. \an Beneden.) 



been derived from a meiublastic type with abundant yolk like that 

 of the bird, and some Mammals living in Australia at the present day 

 still possess eggs of this type. In the higher Mammalia the yolk has 

 disappeared, as it is no longer needed, Ihe embryo, as we have seen, 

 Ixing nourished by means of a placenta, which will be described pre- 

 sently. The early processes r,f development are therefore somewhat 

 peculiar, and though the segmentation is holoblastic, the subsequent 

 development is csscniially similar to thai cf Ihe bird, the embryo 

 beginning lo ap|.eiir in a mass cf ,ells (iMy ,46, /,,-) allaehed lo 



