64 J. W. JENKINSON. 



respect it agrees with those of Selenka (27) and Duval (11), 

 but in his refusal to recognise a layer of trophoblast outside 

 the yolk-sac, as well as in his account of the mode of 

 development of the latter, and of the epiblastic knob, and of 

 their relations to one another, he diifers widely from them. 

 I can only say that the study of my own preparations, quite 

 apart from more general considerations, has convinced me 

 that Selenka and Duval are right, though Duval's ngures 

 are somewhat diagrammatic, and that the interpretation 

 Robinson has put upon his sections is absolutely incorrect. 



The formation of the primitive layers is not so different in 

 the mouse to what it is in other mammals, as Robinson's 

 description would lead us to suppose. In a young blastocyst, 

 such as he has figm'ed, there are present (1) an outer layer, 

 one cell deep, of trophoblast, which is continuous over (2) an 

 inner mass which becomes differentiated into the embryonic 

 epiblast and the hypoblast^ and which is quite distinct from 

 the overlying trophoblast, as my specimens invariably show 

 (see especially figs. 1, 2, 3, 4, and 5, the last of which is cut 

 transversely). At a certain stage this proximal trophoblast 

 (the so called Rauber's cells of the rabbit) certainly becomes 

 very thin (figs. 6 and 7), but it never wholly disappears, and 

 soon thickens again (fig. 8) to form the " Trager," or, to use 

 a modern expression, trophoblastic syncytium, which is 

 destined to play an all-important part in the formation of the 

 placenta. Up to this point the development of the blasto- 

 cyst consists merely in the multiplication of the cells of all 

 layers, in the separation of the hypoblast, which commences 

 to grow round and across the inside of the trophoblast, from 

 the embryonic epiblast, which is now rounded off into a 

 definite spherical or ellipsoid mass, in the flattening of the 

 trophoblastic cells, and the considerable enlargement of the 

 blastocystic cavity. This agrees essentially with the account 

 of Selenka (27), Duval (11), and Cristiaui (10), as may be seen 

 by a reference to their figures (Selenka, figs. 1, 2, 3, 6, 7, 9, 

 10; Duval, figs. 73—80, 83, 84; Cristiani, figs. 21, 24, 26,28, 

 31), which call for little comment except that Duval does not 



