Haddon — Note on the Blastodermic Vesicle of Mammals. 5-11 



this) in which the yolk is still present, and the yolk is partially 



Fig. 2. 



Diagramatic transverse section through the blastoderm and yolk of the ovum of a 

 hypothetical primitive mammal. £"/. embryonic epiblast; e/'. non-embryonic 

 epiblast, i. e. the epiblast of the area opaca ; hy. hypoblast ; y. yolk. 



surrounded by the yolk blastoderm, the blastoderm itself consist- 

 ing of an epiblastic layer and lower-layer cells. In fig. 3 the 



Fig. 3. 



Diagramatic transverse sec- 

 tion of a hypothetical mam- 

 mal's ovum, at a stage cor- 

 responding to Fig. I. The 

 yolk of the .primitive mam- 

 malian ovum is now lost. 



Fig. 4. 



A later stage than fig. 3. The 

 non-embryonic epiblast ep''-. 

 has now grown over the em- 

 bryonic area to form the 

 covering cells. 



Lettering as above— j/. .s. yolk 

 sac. 



yolk is supposed to have been lost, and owing to its absence the 

 yolk blastoderm, i.e. outer cells, have completed the bkstodermic 



Fig- 5- . 



Diagramatic section of a mammalian blastodermic vesicle, in which the primitive in- 

 vagination of the blastoderm is rectified, and the covering cells have extended 

 over the blastoderm. Lettering as before. 



vesicle, and the blastoderm has sunk within the vesicle. This prac- 



