THE EAELY DEVELOPMENT OF THE MAKSUPIALIA. 51 



of the bliistocyst wall, separated by the sutural line, differ 

 somewhat in tlieir characters. On one side of the line (fig. 

 42, tr.ect.) the cells appear to be on the whole slightly larger, 

 and of more uniform size than they are on the other, and they 

 also stain somewhat more deeply. Comparison with later 

 blastocysts shows that the region of more uniform cells is 

 non-formative, that of less uniform, formative. At this stage, 

 however, the differences between the cells of the two regions 

 are as yet so little pronounced that it is practically impossible 

 in the absence of the sutural line to say to which hemisphere 

 an isolated piece of the wall should be referred. 



I am inclined to regard the sutural line in these vesicles as 

 being in course of differentiation, and judging from the dis- 

 position of the cells on either side of it, I think its appearance 

 is to be correlated with the marked increase in the mitotic 

 activity of the cells of the two hemispheres which sets in in 

 vesicles of 3-4 mm. diameter. The preliminary increase in 

 size of the blastocyst up to about the 3 mm. stage might be 

 described as of a passive character, i.e. it does not appear 

 to be effected as the result of the very active division of the 

 wall-cells, but is characterised rather by a minimum of mitotic 

 division and a maximum of increase in surface extent of the 

 cells, due to excessive stretching consequent on the lapid 

 imbibition of uterine fluid. Once, however, the requisite size 

 has been attained, the cells of the unilaminar wall commence 

 to divide activel}^, and doubtless as the outcome of that 

 wave of activity, the sutural Hue makes its appearance 

 between the two groups of independently growing cells. 



On the inner surface of the blastocyst wall, especially in 

 the region of the formative hemisphere, there are present 

 in these vesicles numbers of small deeply staining cells of 

 spherical form, and containing osmicated granules similar 

 to those in the wall-cells. They may occur singly or in groups, 

 and appear to me to be of the same nature as the internal cells 

 of the earlier blastocyst. In addition to these cells, there are 

 present clusters of cytoplasmic spheres, staining similarly to 

 the spherical cells, and apparently of the nature of fragmenta- 



