384 ARTHUR ROBINSON. 
III. The Separation of the Invaginated from the 
Non-invaginated Portion of the Roof. 
When the invagination is completed (fig. 6, Pl. XXIII) the 
inner mass is attached to the external layer by a constricted 
pedicle which apparently becomes broader (fig. 7, Pl. XXIII), 
occupying about half of the superficial area of the roof of the 
blastodermic vesicle. Subsequently the superficial layer of the 
central area is separated by delamination from the inner mass, 
but remains continuous with the peripheral portion of the roof, 
and thus at the commencement of the seventh day the original 
roof of the blastodermic vesicle is separated into an outer layer 
(7) and an inner oval mass (H, fig. 8, Pl. XXIII). 
Therefore at the commencement of the seventh day the 
ovum consists of three distinct portions : 
1. A comparatively large yolk-sac, with walls of irregular 
thickness and a cavity which is traversed and divided by 
nucleated protoplasmic strands. Both the walls and the 
internal reticulum of the yolk-sac are formed by hypoblast 
(HY). . 
2. An oval mass of nucleated protoplasm which rests upon 
and slightly invaginates the middle ova of the proximal wall of 
the yolk-sac. This is the rudiment of the epiblast of the 
embryo and the amnion. It will be termed in the further 
description the epiblast (£). 
3. A nucleated layer of protoplasm which covers the epiblast 
and, extending beyond it, rests upon the peripheral area of the 
proximal end of the yolk-sac. This layer is the trophoblast 
(T); it takes no part in the formation of the embryo or amnion, 
but is developed into a portion of the placenta. 
During the seventh day the uterine epithelium entirely dis- 
appears from the wall of the crypt in which the ovum lies, 
over an area corresponding to the lateral surface of the ovum, 
and both the trophoblast and the hypoblast of the ovum become 
intimately attached to the maternal submucosa. The ovum 
rapidly increases in size, and the inversion of the layers is 
