228 



EMBRYOLOGICAL TYPES 



they meet and fertilise the egg. Fertilisation is therefore 

 internal, as in the chick. 



Cleavage. — Cleavage is total and gives rise to a ball of cells, 

 or morula. A cavity appears within it, and it soon becomes 

 differentiated into an outer layer and an inner mass of cells. 



Fig. 108. — Lepus : early stages in the development of the rabbit. (After 



Assheton.) 

 A, two-cell stage, enclosed by the zona pellucida (zr) ; B, morula (m) 

 stage ; C, blastocyst showing the differentiation into the trophoblast (t) and 

 the inner mass (im) ; D, the inner mass has become the embryonic plate and 

 is differentiated into ectoderm (ec) and endoderm {en) ; E, the trophoblast 

 overlying the embryonic plate — the cells of Rauber (cR) — disappear ; F, 

 after the disappearance of the trophoblast over the embryonic plate ; G, 

 transverse section through the primitive streak (ps). c, coelom ; me, 

 mesoderm. 



The former is called the trophoblast, and the whole structure 

 is known as a blastocyst. 



Implantation. — The lining of the uterus has on its meso- 

 metrial side (see p. 276) a pair of prominent folds, which pro- 

 ject into the cavity or lumen of the uterus. To these, the 



