8 CARL GOTTFRIED HARTMAN 



Following this stage there is a period of growth during which 

 the blastocyst maintains the unilaminar condition. The polar 

 differentiation previously noted now disappears, for the cells of 

 the entire blastocyst are found to have the same structural 

 appearance both in surface view and in section (text fig. 4 7). 



It is not until the blastocysts have attained a diameter of 

 3.5 mm. that the polarity again becomes evident structurally. 

 At this time the cells of the formative ectoderm become differ- 

 entiated, and a little later the entoderm formation begins. The 

 upper formative and lower non-formative areas, of which the 

 latter is usually the larger, are soon separated by a 'sutural line,' 

 The formative cells on one side of the line are usually larger and 

 more lightly staining; those below the line are smaller, more 

 uniformly distributed and possess dark perinuclear zones. 



The next stage is that of entoderm formation, which occurs 

 in blastocysts a little more than 3.5 mm. in diameter. The 

 entoderm arises by a proliferation of modified cells of the forma- 

 tive entoderm, the 'entodermal mother-cells.' These appear at 

 various points in the formative region and seem both to migrate 

 out of their position in the epithelial layer of the entoderm and 

 then multiply, and to maintain their position and give off ento- 

 dermal cells by proliferation. These cells as well as the daughter 

 cells differ markedly in their darker staining qualities from the 

 surrounding ectodermal cells. The young entodermal cells send 

 out pseudopodia-like prolongations, which unite and form a 

 fenestrated entodermal layer. This later closes up to form the 

 primitive entoderm, which spreads until it lines the entire blasto- 

 cyst cavity. . 



It should also be noted that the author homologizes the non- 

 formative portion of the marsupial blastocyst with the tropho- 

 blast of the Eutheria, the embryo of the former, therefore, being 

 without trophoblastic covering. 



The work by Professor Hill here reviewed is the first connected 

 account, supported by a sufficient number of stages, of the early 

 development of a marsupial. It renders Selenka's views unten- 

 able and makes questionable some of the stages found by him. 



