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Mr. A. Fraser. On the Inversion of the [Dec. 21, 



nipped in the middle by a circular constriction, the edges of which 

 ultimately meet, and thus the once single epiblastic cavity is now- 

 divided into two (fig. 3). One at the free pole of the ovum becomes 

 the neuramniotic cavity, N.a. t the other next the placental pole of the 

 ovum is what may be termed the false amnion cavity, f.a. 



It is at this stage that the mesoblast first makes its appearance, 

 budding off from the epiblast at the hinder end of the embryonic 

 area, and spreads rapidly over this area in the form of two lateral 

 plates, which are not continuous across the middle line ; but in 

 addition to this embryonic portion, the mesoblast also spreads in 

 another direction, splitting at the anterior and posterior ends of the 

 embryonic area. One part of it passes over the amniotic part of the 

 neur-amniotic cavity, the other passes internal to the hypoblast, over 

 the free surface of the epiblastic wall (that surface adjacent to the 

 true amnion) of the false amnion cavity. (Fig. 3, m.) 



This stage in the course of another day is succeeded by one in which 

 the two halves of the epiblastic vesicle are separated by a space, which 

 Dr. Thomson suggests, may be termed the interamniotic space (fig. 4, 

 «.a.), while the upper wall of the false amnion cavity has begun to 



Fig. 3. 



Section of ovum from rat, 8 days 16 hours. 



be depressed towards the lower, encroaching upon and filling up its 

 own cavity. (Fig. 4.) This encroachment goes on more and more 

 until finally the walls approach and fuse with one another, the cavity 

 disappearing, and the walls of the false amnion are converted into a 



