THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 61 



regard as an entodermal mother-cell. Cells of this type are 

 not infrequently met with in sections; they usually stain 

 somewhat deeply, and are often found in mitosis. 



The evidence obtainable from the study of in to to pre- 

 parations conclusively pi'oves that some at all events of the 

 primitive entodermal cells are actually derived from the ento- 

 dermal mother-cells much in the way suggested above, whilst 

 others of the primitive entodermal cells are directly formed 

 from mother-cells which bodily migrate inwards. 



Fig. 6b, PL 6, represents a small portion of the formative 

 region of an '04 vesicle viewed from the inner surface. In 

 the centre of the figure, surrounded by the larger, lighter 

 staining (ectodermal) cells of the wall, is a smaller cell in the 

 telophases of division, the cytoplasm of which is granular and 

 stains deeply. That cell unmistakably forms a constituent of 

 the unilaminar wall. I regard it as an entodermal mother- 

 cell. Fig. iJQ shows another cell of the same character in the 

 anaphases of division, which likewise forms a constituent of 

 the unilaminar wall, but which differs from the corresponding 

 cell in fig. 65 in that its cytoplasmic body has extended out 

 on one side (lower in the figure), so as to directly underlie 

 part of an adjacent ectodermal cell. In other words we have 

 here a surface view of the condition represented in section in 

 fig. 7b, only the entodermal mother-cell depicted therein is not 

 actually in process of division. Fig*. 67, taken from the same 

 preparation as fig. 65, shows what I take to be the end result 

 of the division of such a cell as is represented in the two 

 preceding figures. Here we see two small deeply staining 

 cells towards the centre of the figure, which from their dis- 

 position and agreement in size and cytological characters 

 are manifestly sister-cells, and the products of division of 

 just such an entodermal mother-cell as is represented in fig. 

 65, or, better, fig. 66. The one cell (upper in the figure) is 

 more angular in form and manifestly still lies in the uni- 

 laminar wall; the other (lower in the figure) is ovalish in form 

 and is no longer a constituent of the unilaminar wall, but is 

 on the contrary a free cell, definitely internal both to the 



