236 RICHARD ASSHETON. 



been cut, if it is only one layer in thickness. The diagram 

 Vj is derived from fig. 6 of van Beneden and Julin^s work, or 

 from fig. 32 of Duval's work. 



If there is such a distinct difference in colour between the 

 outer layer and inner layer of the inner mass, as shown in 

 Duval's figures, my interpretation can hardly hold good. 

 Duval, however, explains that his figures are to a certain 

 extent diagrammatic. He writes on p. 134, " Sur nos figures, 

 nous avons exagere et schematise ces differences d'aspect entre 

 les segments petits et clairs (cellules ectodermiques) et les 

 segments plus fences, plus granuleux (cellules endodermiques), 

 et a cet effet nous avons, d'une maniere conventionelle, donne 

 aux cellules ectodermique un corps clair, avec un noyau fonce 

 et aux cellules endodermiques un corps fonce avec noyau clair, 

 de fa9on a permettie desaisirau premier coup d'oeil la maniere 

 dont se comportent ces cellules les unes vis-a-vis des autres 

 dans les stades successifs que nous figurons." 



Another extremely important stage from Duval's point of 

 view is that of bis fig. 36. 



Here the outer layer consists " generalement d'une seule 

 couche de cellules cubiques." At the embryonic pole, however, 

 these cells tend to become arranged in two layers. It is, 

 however, a very slight thickening. The inner mass is very 

 much flattened, and extends halfway round the walls of the 

 vesicles. 



At the embryonic pole the inner mass is, although distinct 

 from the outer wall, two layers thick, which thickness, however, 

 extends over a short area only. 



Duval assumes that the whole of this inner mass is hypo- 

 blast, and that it eventually becomes completely flattened out 

 into a single layer, as he shows in his diagram B on p. 106 

 (vol. 1896). But he apparently has not found this stage — it 

 is not shown in any of his figures. 



May we not, on the other hand, suppose that in the double- 

 layered condition of the inner mass of this stage the inner 

 layer is the true hypoblast, and the cells lying between this 

 and the outer layer are the true epi blast cells? May not 



