DEVELOPMENT OF THE PIG DUEING FIRST TEN DATS. 341 



changed. Previously they contained many vacuoles, but now 

 they are without vacuoles^ and far more homogeneous in 

 their minute structure. There are no longer any thinly 

 drawn-out portions such as can be seen in fig. 19. 



The inner mass has undergone a complete change. Instead 

 of being a compact lenticular mass, it is now divided sharply 

 into (a) a small group of cells, very often lying quite separated 

 from the outer layer of cells, (b) a loose layer or network of 

 cells lying apparently but very slightly attached to the inner 

 surface of the group (a), and extending beyond its limits on 

 to the *' outer layer ^^ (figs- 21 and 24). ■ So slight is the con- 

 nection between {b) and {a), that very often the two appear to 

 be entirely separated (v. figs. 22 and 23,25 and 26). It seems 

 probable that the conditions of figs. 21 and 24 are normal, 

 and that the separation of the two layers seen in figs. 22, 23, 

 and 25, 26, may be due to the disturbances during preserva- 

 tion. It may therefore be accidental, but the specimens which 

 show the separation were to all appearances quite perfect when 

 they were examined after having been stained and cleared. 

 In any case it seems to point to a very slight connection 

 between those two layers. This is curious because in the 

 immediately preceding stages (figs. 19, 20) there is hardly 

 anything which indicates it. Fig. 19 shows a whiter core, E., 

 which may perhaps suggest such a separation. 



I shall throughout this paper talk of these three separate 

 cell areas as trophoblast, epiblast, and hypoblast, but in 

 adopting Hubrecht's terminology — which, as he remarks, is 

 extremely convenient— I do not adopt the conclusions to 

 which he comes regarding the homology of the trophoblast as 

 enunciated in his work, ' Die Phylogenese des Amnions und 

 die Bedeutung des Trophoblastes.' In the dissertation fol- 

 lowing my description of the sheep's development (la) I give 

 reasons for regarding the trophoblast as really part of the 

 hypoblast. So we may describe the embryo, shortly after 

 the loss of the zona radiata, as a closed vesicle of very varied 

 shape, whose wall is formed by a single layer of cells which are 

 nearly cubical in shape. This layer is the trophoblast. At one 



