30 



ENTYPY OF THE GERMINAL AREA 



closed from the first. Its floor becomes the embryonic ectoderm, its roof the 

 amniotic ectoderm, and this is attached to the trophoblast by a short stalk, 

 which may be termed the amnion-stalk. Fig. 40, which represents an attempt 

 to visualise this hypothetical stage of the human ovum, will make these statements 

 clear ; but for a full understanding of the points at issue, a comprehension of the 

 phenomenon known as entypy of the germinal area is necessary. 



It must be stated here that the view adopted in the text has not the assent of all embryo- 

 logists. It is clear that the early blastoderm is invaginated in the human ovum, but is the 

 invagination primary as described above, or is it secondary ? It is held by some that, owing to the 

 complete imbedding of the ovum in the decidua, the embryonic ectoderm, at first on the surface 

 of the blastocyst, is very early closed in by precocious amnion folds. As a further result, the 

 growth of the blastoderm causes it to be inverted into the cavity of the vesicle, and the stage 

 imagined in fig. 40 would be reached by the fusion of the folds over the embryonic area to form 

 what has been named the amnion-stalk. Certain observations of Selenka on Hylobates embryos, 

 and of Mall on abnormal human ova, support this view, which has also been advocated by 

 Keibel. ' The view adopted in the text is that of Van Beneden, Selenka, Hubrecht, and others, 



a 



FlG. 42. DlAGBAMS TO ILLUSTKATE ENTYPY OF THE GEBMINAL AREA. (T. H. BryCG.) 



Blastodermic vesicle : a, of rabbit ; b, of mole ; c, of bat ; d, of mouse or rat ; e, of guinea-pig ; /, of 

 kalong. 



Trophoblast represented by continuous black lines or masses ; entoderm by interrupted lines ; 

 embryonic ectoderm by epithelial cells. 



and it differs from the first only in holding that from the circumstances of the complete 

 imbedding of the ovum the embryo-amnio-genetic ectoderm is invaginated at a still earlier stage, 

 the blastocyst wall being folded over the formative ectoderm even before it is differentiated 

 into embryonic ectoderm. 



Entypy of the germinal area. In the rabbit ovum at this stage the 

 trophoblast is reduced to a thin sheet of flattened cells, against which the cells of the 

 formative cell-mass spread themselves out at an early stage (fig. 36, d, p. 26). 

 After the formation of the entoderm, the cells between that layer and the tropho- 

 blast form a plate of columnar cells, the embryonic ectoderm (fig. 41, a), which is 

 flat from the first, and directly applied to the covering layer of trophoblast (here 



