306 STUDY OF THE BLASTODERMIC VESICLE. 



stained before they are imbedded. For staining, alum cochineal or borax car- 

 mine 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. Un- 

 fortunately 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 em- 

 bryonic 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 vesi- 

 cles 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 

 examinations the so-called live-box, such as was formerly much used by micro- 

 scopists 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 bounda- 

 ries 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 dur- 

 ing 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 en- 

 velope, which in the rabbit ovum during segmentation is very thick and con- 

 spicuous, 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 identical in character with the villi which have been observed 



