STUDY OF RABBIT BLASTODERMIC VESICLES IN ALCOHOL. 167 



is recommended. Finally, the specimens are to be imbedded in paraffin and cut 

 in series in the usual manner; sections of from 6 to 8// are desirable. Unfor- 

 tunately, no method has yet been devised by which these delicate vesicles may be 

 imbedded without distortion of their form, so that, when the sections are finally 

 obtained, the blastodermic walls are wrinkled and more or less out of shape. But 

 fortunately, owing apparently to its greater thickness, the embryonic area usually 

 escapes distortion and appears in the sections of normal form, or nearly so. 



Study of Rabbit Blastodermic Vesicles in Alcohol. 



All of the most important points in the structure of the blastodermic vesicles 

 of the rabbit from the fourth to the seventh day may be fairly well observed by 

 examining the hardened vesicles in alcohol under the microscope. For such exami- 

 nations the so-called live-box, such as was formerly much used by microscopists 

 for the study of living creatures, will be found very convenient. Care must be 

 taken to have plenty of alcohol around the specimen and not to lower the cover 

 so much as to exert any pressure upon the vesicle. It is not difficult to place the 

 vesicles so that any part of their surface may be examined with a No. 7 objective. 

 In the uncolored specimen the nuclei and even many of the boundaries of the 

 cells can be clearly made out. 



In the following descriptions ages have been chosen at which the important 

 characteristics can usually be observed. The variation is so great in range during 

 early stages that the development described below for a given age is often found 

 in older or younger specimens, and specimens of a given age may exhibit a less 

 or a more advanced stage of the embryonic formation than is here put down 

 for that age. In general the correspondence of the stage of development to the 

 size of the vesicle is more exact than to its age. 



Vesicles at Five Days (5 X 24 hours). At this age the vesicles are always found 

 in the upper portion of the uterus. Sometimes all of those in one uterus are quite 

 close together, at other times somewhat scattered and lying singly. The vesicles 

 are extremely variable in size, for they measure from 0.6 to 0.9 mm. They are 

 spherical or nearly so, and are surrounded by a thin membrane, which in reality 

 corresponds to both the zona pellucida and the outer albuminous envelope, which 

 in the rabbit ovum during segmentation is very thick and conspicuous, but which 

 is always extremely thin when the stage of the blastodermic vesicle is reached. 

 Upon the outside of this really double membrane appear a certain number of 

 small villus-like projections, which are highly refringent. They are probably identi- 

 cal in character with the villi which have been observed upon the ovum of 

 the dog (page 45), but are smaller in all of their dimensions. Immediately un- 

 derneath the external membrane there is a continuous layer of cells belonging 

 to the ectoderm and extending completely around the ovum. The layer is some- 

 times designated specifically as the " outer layer" or as the " subzonal layer" It 

 also extends over the embryonic shield; the portion upon the shield is often termed 



