BY JAS. P. HILL AND C. J. MARTIN. 51 



form by folding of the l^lastoderm is enabled to occur l^y the 

 embryo making room for itself, so to speak, by the using up of 

 the fluid contents of the vesicle. 



Against the view here put forward, it may be urged that in 

 certain other forms there is a similar rapid increase in size of the 

 I )lastodermic vesicle by the absorjation of fluid, and yet there is 

 no retardation in development. In Didelphys, for example, the 

 1 )histodermic vesicle, according to Selenka, increases in one day 

 from a diameter of 6 mm. to one of 15 mm., and at the end of this 

 time the embryo is folded oflf, the medullary groove is closed and 

 the amniotic folds developed. All these processes can, how^ever, 

 easily occur on an expanding blastodermic vesicle lying naked in 

 the cavity of the uterus and devoid of any such mechanical 

 obstacle as would be presented by the presence of a resistent shell 

 membrane. 



Ectoderm. 



The ectoderm forms a continuous covering for the whole of the 

 l)lastodermic vesicle. It consists, except in the regions to be 

 subsequently mentioned, of a single layer of polygonal cells. Over 

 the greater portion of the embryonic area the cells are much 

 flattened, while in the head region of the embryo and in the 

 extra-embryonic region of the wall of the vesicle they appear 

 cubical in section. 



Medullary plate. — The medullary plate is, as already 

 mentioned, still practically flat. Medullary folds are only present 

 in the anterior region of the future fore-brain ; their appearance 

 in this region is probably to be associated with the very early 

 appearance of the optic grooves. The plate consists of elongated 

 cells, the nuclei of which are situated at different levels simulating 

 the appearance of several layers of cells. The lateral portions of 

 the plate are thickest, and are connected by a median much 

 thinner portion which sends down a keel-shaped process in 

 some parts to meet the notochord. Along the median portion of 

 the plate there runs a distinct groove — the " Riickenfurche." 

 Beiiinninf; as a shallow t^roove slightlv behind the anterior end of 



