48 CARL GOTTFRIED HARTMAN 



In the forty-four preparations of young opossum blastocysts 

 in my possession, taken from four different females, only five do. 

 not show clear cases of included cells with nuclei and in three of 

 these cases I am in doubt as to whether or not chromatin frag- 

 ments are present. If, therefore, cell-inclusion constitute an 

 abnormality, my forty-four blastocysts are only 10 per cent or 

 less normal. This I am certain is not the case. 



There is, of course, every gradation in the volume of included 

 material: lipoid, cytoplasmic and chromatic. There may be 

 only a few fat grains and lumps of cytoplasm (fig. 25 and text 

 fig. 5 D) or a single perfect cell in addition to these amorphous 

 masses (fig. 26). Other eggs have two, three or a dozen small 

 cells, or, as many or more large cells which may finally almost 

 completely fill the segmentation cavity (fig. 5). One egg, indeed, 

 is practically a solid morula, the cavity being all but obliterated 

 by cell masses. But the included cells are fragmenting and the 

 whole structure has the appearance of degeneration. This and 

 other abnormal eggs are listed as such and are not among the 

 forty-four on which the description of this stage is based. All 

 of these forty-four I am convinced are normal and they are 

 among the best fixed and stained preparations in my possession. 



For three reasons, therefore, the conclusion is fully justified 

 that blastocysts containing a limited number of cells within their 

 cavities are perfectly normal, but that these cells have no morpho- 

 logical significance: 1) They are of undoubted occurrence in 

 thirty-nine out of forty-four blastocysts of approximately the 

 50-celled stage, which are normal and the walls of which do not 

 differ essentially from blastocysts apparently free of included 

 nuclear material. The three cases of similar inclusions of cells 

 observed by Selenka should also be noted in this connection. 

 2) Included cells without morphological significance have been 

 observed in cleavage stages in other mammals. 3) Later stages 

 of Didelphys and Dasyurus, apparently normal in every respect, 

 show fragments of cells not entirely digested or absorbed. 



To recapitulate, the blastocyst of the opossum, formed in the 

 16-celled stage, is completed soon after the fifth cleavage or at 

 about the 40-celled stage. At this time or a little later a polar 



