BLASTOCYST ""7 



however, never extends to the ventral side of the yolk sac, 

 whose endoderm is therefore, in that region, covered only by 

 ectoderm (trophoblastic) : eventually the ventral wall thus formed 

 breaks up and disappears. Meanwhile, outgrowths or villi of 



om.f 



mm 



YiG. 522.— Sections of a rabbit embryo on the ninth day.— After various authors. 



A Longitudinal section through a blastocyst removed from the uterus ; B, transverse section of the 

 ' uterus and of a blastocyst in st^u.— Partly after Marshall. 



al. Allantois ; am.f., amnion fold ; c, coelom ; c.p., pericardium ; cm circular ™"scles of uterus ;i^.y., 

 ' dorsal w4ll of yolk sac ; e., embryo ; h.f., head fold ; hyp., endoderm of embryo -.hyP ., endoderm of 

 yofk sac ; /.m., longitudinal muscles of uterus ; l.u., lumen of uterus ; mes., pesoderm ; »?"»•. n^eso" 

 metrium of uterus^ n.f., neural folds ; t.f., tail fold ; tr., trophoblast (grey m A, black m B); tf., 

 Sen^d re^on of samVin which will a^si placenta ; tr.v., viUi of trophoblast ; u.c, utenne capillaries; 

 v.w.y., ventral wall of yolk sac ; y.s., yolk sac. 



