416 WALTER HEAPE. 



those of other animals, yet the further development proceeds 

 (up to a certain point) the more strikingly similar these condi- 

 tions become, and the usual rule that embryos of various 

 animals differ from one another less in their earlier than in 

 their later stages of development is therefore here reversed. 



In concluding this section I would draw attention to the facts 

 treated fully below (1) that the central position of the inner 

 mass of segmentation spheres in both the rabbit and the mole 

 is merely temporary, and that subsequently these cells, with 

 the exception of a very small number, form a portion of the 

 wall of a vesicle, the " blastodermic vesicle." 



(2) That the so-called blastopore (Beneden) cannot be simi- 

 lar to the blastopore present in Amphioxus, and has merely a 

 secondary origin, its existence being caused by the temporary 

 involution of a portion of the wall of the blastodermic vesicle. 



Stage B. 



The Blastodermic Vesicle and the Formation of the 

 Hypoblast and Epiblastofthe Embryo. 



The conversion of the fully-segmented ovum into the so- 

 called blastodermic vesicle takes place shortly after the appear- 

 ance of the ovum in the uterus. It is due partially to a 

 flattening-out of the cells of the outer layer, and partially to 

 the conversion of certain of the cells of the inner mass into outer 

 layer cells. 



The result of these changes is a vesicle the wall of which is 

 composed of, for the most part, a single row of flattened cells, 

 the much attenuated zona radiata surrounding the whole. 



In the course of its growth the vesicle becomes so large that 

 the wall of the uterus in the region where it is placed is dis- 

 tinctly swollen. 



It is clearly impossible for the delicate-walled ovum to 

 exi)and in the form of a vesicle, and distend the uterine walls 

 by virtue of the growth of its cells; it must be therefore con- 

 cluded that it obtains some support. This support is rendered 

 from within. 



