DEVELOPMENT OF THE OPOSSUM 47 



one or two may be present, in yet others numbers of them may occur. 

 They may be definitely nucleated, but this is exceptional; more usually 

 they contain one or more deeply staining granules (of chromatin?), or 

 are devoid of such. They are of no morphological importance, and 

 I think there can be no doubt that they represent cells or fragments 

 of cells which have been separated off from the cellular wall during the 

 process of active growth. They are of common occurrence in later 

 blastocysts, and it is possible the so-called 'yolk-balls' observed by 

 Selenka in Didelphys are of the same nature.'^ 



It is thus seen that Hill does not consider his later blasto- 

 cysts abnormal, no doubt because of the large proportion of speci- 

 mens containing included cells. 



Again, in later full-grown bilaminar blastocysts Hill finds 

 islands of deep-staining cells, both in the formative and the non- 

 formative areas, in contact with the blastocyst wall. These 

 cells have large nuclei and may be actively proliferating or, on 

 the other hand, they may be in process of degeneration. He 

 considers them ''of no morphological importance and (they) 

 are destined sooner or later to degenerate." I have not seen 

 cells like these last described, but Selenka mentions similar ecto- 

 dermal growths in opossum vesicles containing embryos with 

 three somites. 



It is not amiss at this point to recall that cell fragments have 

 been reported in normal cleavage stages of Eutherian embryos, 

 e.g. by Assheton ('98) in the sheep. In the 8- to 17-celled stages 

 of the sheep ovum the segmentation cavity ''always contains 

 spherical or otherwise shaped masses of substance in every way 

 similar to the cytoplasm of the segments." In three specimens 

 chromatin spots were recognized, in others chromatin-like gran- 

 ules. Of these Assheton says: ''I cannot give any explanation 

 of the fragments of cells seen within the segmentation cavity, 

 nor of the origin of the nucleus-like body."^^ 



These cases of included cells or cell fragments within the 

 cleavage cavity as described by Selenka, Hill and Assheton for 

 Didelphys, Dasyurus and Ovis respectively, are cited as prob- 

 ably illustrating cases of such inclusions in normal embryos. 



" Loc. cit., p. 46. 

 '1 Loc. cit., p. 212. 



