456 W. B. SCOTT AND HENRY F. OSBORN. 



error which involves some of his subsequent conchisions in 

 doubt. 



Having thus briefly considered the origin of the two inner 

 layers, as related to the phenomena of invagination, we shall 

 return to the history of the epiblast from the beginning, and 

 resume our discussion of the mesoblast and hypoblast in the 

 subsequent pages. 



General Features of the Epiblast. 



When the epiblast can first properly be said to be formed, 

 it consists of a single layer of very large quadrate cells, with 

 large clear nuclei. In the next stage, when the invagination 

 first commences, the cells have somewhat lengthened out, 

 but are still very broad (Plate XX, fig. 1). When the in- 

 vagination has progressed considerably, and the segmenta- 

 tion cavity has been much narrowed, we find that the cells 

 have assumed the condition which they retain for some time 

 after this. They are long, narrow, and columnar ; most of 

 them can be traced through the layer from one surface to 

 the other without any change of size, although here and 

 there several may be seen which have a wedge-shape, and 

 alternate arrangement with their neighbours. The nuclei, 

 however, are arranged in two rows, like those of the Elasmo- 

 branch epiblast. In general appearance, up to this time, 

 the epiblast is more like that of Petromyzon than of any 

 embryo which we have seen,^ but the arrangement of the 

 cells is somewhat more regular. For a short time, indeed, 

 the appearance of the two is almost identical, but in the 

 Newt the cells speedily become narrower, and more columnar 

 in character, and the nuclei assume the alternate arrange- 

 ment which is only faintly indicated in the Lampi-ey. 

 During Stage a, when the medullary groove has begun to 

 make its appearance, the middle line of the dorsal epiblast, 

 exhibits a decided thinning to form the groove (Plate XX, 

 fig. 4). But this grove is not at this period, nor do we find 

 it afterwards, nearly so deep or so wide as it is in the Elas- 

 mobranchs.^ 



The next change of importance takes place during Stage 

 B (Plate XX, fig. 5), when the medullary folds are well 

 formed. These folds are caused by the multiplication of 

 cells of the epiblast, which here becomes much thickened. 

 Although the folds are several cells thick they show no indica- 

 tion of being separated into different layers. With the excep- 



' See a paper by Calberla, ' Morph. Jahrbuch,' 1877, 3, taf. xii, fig. 7. 

 * Balfour, loc. cit., plate iv, fig. 8 a. 



