EARLY MAMMALIAN DEVELOPMENT. 



87 



thinner portion of the wall (Fig. 56). In the opossum (Hartman) no true 

 morula is formed, since during even the earliest cleavage stages the cells are 

 arranged in a layer around a central cavity which in this case contains some 



FIG. 56. FIG. 57. 



FIG. 56. Sections of blastocysts of the white rat, 5 days after insemination. Huber. Note the 

 cavity within the structure and the cluster of cells, comparable to the inner cell mass 

 which forms part of the vesicle. 



FIG. 57. Section of a 1 6-cell stage of an ovum of the opossum. Hartman. Blastocyst formation 

 is anticipated in that the cells are arranged around the cleavage cavity, and will be com- 

 pleted when the gaps (i) are closed. The black masses represent yolk globules in the 

 t. cytoplasm and in the cleavage cavity. 



yolk material (Fig. 57). In Fig. 58 there is shown a section of the developing 

 ovum of the lemur, Tarsius spectrum, in which the covering layer and inner 



FIG. 58. Section of the blastocyst of the lemur, Tarsius spectrum. Hubrecht, from Quain's 

 Anatomy, i, Inner cell mass; 2, trophoderm. 



cell mass are very evident (Hubrecht). The hollow structure illustrated in 

 the four forms above is called the blastocyst (or not quite so correctly, the 

 blastodermic vesicle). It is not regarded as homologous to the blastula of 



