424 WALTER IIEAPE. 



gone a slight change since we last examined it, inasmuch as it 

 now consists for the most part of a single row of columnar 

 cells, which at the sides of the area gradually become less and 

 less columnar and eventually merge into the flattened epiblast 

 cells of the wall of the vesicle. This change is, however, 

 temporary, since in sections of older embryonic areas the epi- 

 blast is again two layers deep (figs. 33, &c.). 



Fig. 30 is a transverse section of the area drawn in fig. 10 ; 

 it is very similar to fig. 28 ; but the edges of the embryonic 

 area in this case appear to end abruptly, the wall of the vesicle 

 having been torn away owing to its close attachment to the 

 uterus. 



The condition of the ovum is now considerably changed 

 from what it was when the blastodermic cavity first appeared ; 

 it may be divided into two areas, the embryonic and non- 

 embryonic areas. The embryonic area is throughout com- 

 posed of an outer thickened layer of columnar epiblast cells 

 which has been derived partially from a portion of the inner 

 mass and partially from outer layer cells, and an inner layer 

 of somewhat rounded hypoblast cells derived entirely from 

 cells of the inner mass. The non-embryonic portion of the 

 ovum may in its turn be divided into two regions. 



First, the region immediately surrounding the embryonic 

 area which is formed of two layers, an outer of flattened outer 

 layer cells now known as epiblast cells, continuous with the 

 epiblast of the area, and an inner of flattened hypoblast cells 

 continuous with the same layer in the embryonic portion of the 

 ovum. 



Secondly, the region situated at the opposite pole of the 

 ovum to the embryonic area, where a single row of flat epiblast 

 alone exists. 



Historical. — The details attending the formation of the epi- 

 blast has given rise to a considerable amount of discussion. 

 According to Edward van Beneden (No. 5) the fully-segmented 

 ovum of the rabbit develops into the blastodermic vesicle by a 

 multiplication and a flattening of the outer layer cells, the inner 



