THE EARLY DEVELOPMENT OF TUL MAKSL'i'lALlA. 09 



embryonal area, however, its peripheral extension below the 

 extra-embryonal ectoderm is much less easily separable in the 

 intact condition (cf. fig. 50), because of its greater delicacy 

 due to the fact that it has here largely the form of a cellular 

 reticulum. In this extra-embryonal region the entodernial 

 cells are frequently found in mitosis. It would appear, then, 

 that the entoderm is first laid down in the region of the em- 

 bryonal area as a cellular reticulum, which later becomes 

 transformed into a continuous cell-membrane, and that its 

 jieripheral extension over the inner surface of the extra- 

 embryonal ectoderm is the result of the growth and activity 

 of its own constituent cells. 



This peripheral growth continues until there is formed 

 eventually a complete entodermal lining to the blastocyst 

 cavity. The rate of growth appears to be somewhat variable. 

 In a series of primitive streak vesicles (6-6*75 mm. in diameter) 

 the lower third of the wall is, I find, still unilaminar. In 

 another series of vesicles of the same developmental stage 

 (4*5-6 mm. in diameter) a unilaminar area is present at the 

 lower pole, varying from i x *5 mm. m diameter to as much 

 as 4 mm. Even in vesicles 7-7'5 mm. in diameter a uni- 

 laminar patch may still occur at the lower pule, but in vesicles 

 8'5 mm. in diameter (stage of flat embryo) the entodermal 

 liiiino- appears always to be complete. 



The Origin of the Entoderm in Eutheria. — The 

 remarkable facts relative to the origin of the entoderm in 

 Dasyurus which I have been able to place on record in the 

 preceding pages, thanks to the large size attained by the 

 blastocyst prior to the differentiation of the formative germ- 

 layers and to the circumstance that the formative cells are 

 not arranged, as they are in Eutheria, in the form of a more 

 or less compact cell-mass, but constitute a thin undaminar 

 cell-layer of relatively great extent which can easily be cut 

 up with scissors, and which, after staining and mounting on 

 the flat can be examined under the highest powers, throw, it 

 seems to me, a new and unexpected light on the mammalian 

 entoderm, and at the same time help to fill the considerable 



