664 EMBRYOLOGY I MAMMALS 



the rabbit they take place as follows. The ovum has no vitelline 

 membrane, but is enclosed in a relatively thick striated membrane, 

 the zona radiata, secreted by the ovary, and has outside that a coat 

 of albumen secreted by the oviduct. The morula (Fig. 518, 3) 

 hes in the uterus, which is reached (p. 457) at the end of segmenta- 

 tion. It is covered by a single layer of cells which are rather smaller 

 and more transparent than those within. This layer is known as 



pr hy 



Fig. 518. — Early stages iu the development of the rabbit. — After various authors. 



I. Ovum with polar bodies ; 2, two-cell stage ; 3, morula ; 4, section of a later stage ; 5, section of the yoxing 

 blastocyst ; 6, section of an older blastocyst ; 7, section of the embryonic area after difEerentiation 

 of two layers. 



e.e., Embryonic ectoderm ; i.e., inner cells ; pr.hy., primitive endoderm ; R.c., Rauber's cells ; tr., trophoblast. 



the trophoblast (Fig. 518, 4, tr.), and will form a part of the 

 ectoderm — that part, namely, which covers the false amnion, but 

 not the ectoderm of the embryo proper nor that which lines the 

 amnion. It now begins to absorb water and nutriment secreted by 

 the wall of the uterus and to be distended, so that it separates 

 from the inner mass of cells, except at one side, where they remain 

 sticking to it, at first as a knob (Fig. 518, 5), which afterwards 

 (Fig. 518, 6) flattens out upon the inner side of the trophoblast, 

 forming a circular patch, known as the embryonic plate. This 

 afterwards becomes oval, and will give rise to the embryo. The 

 bladder-hke structure which has thus arisen is known as the 



