THE EAIJLY DEVELOPMENT OP THE MARSUPIALTA. 57 



development, must for the present be left an open question. Of 

 the actual existence in the unilaminar formative region of these 

 ^99 aud '04 blastocysts of two varieties of cells, respectively 

 ectodermal and entodermal in significance, there can be no 

 doubt. In preparations of the formative regiou, however^ 

 whilst one can without hesitation identify certain cells as 

 being in all probability of ectodermal significance and others 

 as prospectively entodermal (cf. figs. 65, 6Q), it must be 

 admitted that one is often in doubt as to whether one is 

 dealing with small ectodermal cells or with genuine ento- 

 dermal mother-cells. It is, therefore, hardly to be wondered 

 at that I have not yet been able to satisfactorily'' determine 

 at what precise period the entodermal mother-cells first 

 become differentiated, though judging from the facts that 

 in the earliest vesicles in which the sutural line is recogniis- 

 able one region of the wall already differs from the other in 

 the less uniform size of its constituent cells, and that internally 

 situated entodermal cells are already present in small numbers 

 in the '01 vesicles (fig. 71), I incline to the belief that it 

 will probably be found to about coincide with the first 

 appearance of the sutural line. To this question I may 

 perhaps be able to return at some future time. 



In addition to the presence of these entodermal mother- 

 cells, which enter directly into its constitution, the formative 

 region of the '99 and '04 blastocysts is characterised by the 

 occurrence on its inner surface of definitely internal cells, 

 which generally agree with the former cells as regards size 

 and staining properties and are evidently related to them. It 

 is these internally situated cells which directly give origin to 

 the definitive entoderm of the later blastocysts, and one need, 

 therefore, have no hesitation in applying to them the designa- 

 tion of primitive entodermal cells. They are exclusively found 

 in relation to the formative hemisphere, and appear in in toto 

 preparations as flattened, darkly staining cells closely applied 

 to the inner surface of the unilaminar wall, and disposed quite 

 irregularly, singly, and in groups. They vary greatly in 

 niimber in blastocysts of even the same batch, but on the 



