FORMATION OF THE LAYERS. 263 



preliminary account 1 it was stated that this was a condition 

 of the mesoblast at a very early period, and that it was probably 

 its condition from -the beginning. Sections are now in my 

 possession which satisfy me that, from the very first, the meso- 

 blast arises as two distinct lateral masses, one on each side of the 

 axial line. 



In the embryo from which the sections PI. 7, fig. 8 a, 86, 

 8c were taken, the mesoblast had, in most parts, not yet become 

 separated from the hypoblast. It still formed with this a con- 

 tinuous layer, though the mesoblast cells were distinguishable by 

 their shape from the hypoblast. In only one section (b] was any 

 part of the mesoblast quite separated from the hypoblast. 



In the hindermost part of the embryo the mesoblast is at its 

 maximum, and forms, on each side, a continuous sheet extending 

 from the median line to the periphery (fig. 8 a). The rounder 

 form of the mesoblast cells renders the line of junction between 

 the layer constituted by them and the hypoblast fairly distinct ; 

 but towards the periphery, where the hypoblast cells have the 

 same rounded form as the mesoblast, the fusion between the two 

 layers is nearly complete. 



In an anterior section the mesoblast is only present as a cap 

 on both sides of the medullary groove, and as a mass of cells 

 at the periphery of the section (fig. 8<5>) ; but no continuous layer 

 of it is present. In the foremost of the three sections (fig. 8^) 

 the mesoblast can scarcely be said to have become in any 

 way separated from the hypoblast except at the summit of the 

 medullary folds (m). 



From these and similar sections it may be certainly concluded, 

 that the mesoblast becomes first separated from the hypoblast 

 as a distinct layer in the posterior region of the embryo, and 

 only at a later period in the region of the head. 



In an embryo but slightly more developed than B, the forma- 

 tion of the layer is quite completed in the region of the embryo. 

 To this embryo I now pass on. 



In the non-embryonic parts of the blastoderm no fresh fea- 

 tures of interest have appeared. It still consists of two layers. 

 The epiblast is composed of flattened cells, and the lower layer 

 of a network of more rounded cells, elongated in a lateral 



1 Loc. cit. 



