378 ARTHUR ROBINSON. 
During the fifth day (figs. 4 and 5, Pl. XXIII) the thin por- 
tion of the wall of the blastodermic vesicle increases very con- 
siderably in extent, its apical portion is projected away from 
the thickened portion of the wall, and thus the long axis of the 
ovum comes to lie at right angles to its former position. In 
the meantime, however, the ovum has altered its position in 
relation to the wall of the uterine cavity. The most promi- 
nent points of its thick and thin walls are no longer opposed 
to the sides of the uterine canal ; they are directed towards the 
ends of that channel, or, in other words, the long axis of the 
ovum lies parallel with the long axis of the uterine canal, and 
there is still no separation of the thick portion of the wall of 
the vesicle into an inner and outer layer. 
During the sixth day important changes take place in the 
relations of the ovum to the uterus, and of the various parts 
of the ovum to each other. The inversion of the layers com- 
mences, and the first traces of the archenteron appear (figs. 6 
and 7, Pl. XXIII). 
The ovum lies in a cylindrical crypt in the thickened 
mucous membrane of the distal (antimesometrial) side of the 
uterine canal. The long axis of the crypt is at right angles 
to the long axis of the uterus, and therefore the ends of the 
ovum occupying the crypt are now distinctly proximal or me- 
sometrial (P, fig. 6), and distal or antimesometrial (D). 
The crypt is lined by uterine epithelium, which is reduced 
to a layer of flat cells where it is opposed to the sides of the 
ovum, with which, however, it is not united, but over the 
remaining parts of the walls of the crypt the epithelium has a 
cubical or low columnar form. 
Fig. 6 is a camera drawing of the seventh section of an 
ovum, which is divided longitudinally into fourteen sections. 
The greatest length of this ovum is 95 uy, and its greatest 
breadth 49 p. 
It is a vesicle. Its distal wall is formed by a multinucle- 
ated mass of protoplasm indistinctly divided into cell areas. 
The protoplasm of the distal pole stains but faintly with car- 
mine, and it corresponds in all essential respects with the 
