46 W. B. SCOTT AND HENRY F. OSBORxV. 



this reason we are forced to the conclusion that even the 

 early condition of the epiblast in the Batrachia is a secondary 

 modification, and that the primitive condition of the layer is 

 single. 



As opposed to this conclusion maybe adduced the fact that 

 in the spinal cord of the Batrachia the two layers at first 

 fuse together and at a later time reappear, as if the double- 

 layered condition were a primary, the single-layered a 

 secondary, and the reappearing double layer a tertiary stage 

 in development. And further, that the first stage has been 

 retained only in the Batrachia and (?) Osseous Fishes, and lost 

 in other known vertebrates. But this appears unlikely, and 

 standing entirely by itself, the above-mentioned fact cannot 

 be considered to have any great value. 



The Hypoblast. 



We shall now continue the history of the hypoblast from 

 Stage A onwards, until the development of the notochord. 

 The embryo at this stage (see PI. IV, fig. 4) is still spherical. 

 In the section figured, which is in the anterior region of the 

 embyro, the alimentary canal is broad and low, lined above 

 by a deep single layer of columnar hypoblast cells. These 

 are broader and longer than the epiblast cells above them, 

 with nuclei of a spherical rather than oval shape. They are 

 in contact with the epiblast broadly across the middle line, but 

 at the sides, just below the two slight folds on either side of 

 the medullary groove, the mesoblast begins to intervene as a 

 single layer of small cells. Beneath these the hypoblast 

 cells lose their columnar shape, and becoming more quadrate 

 are gradually reflected around the sides of the alimentary 

 canal, becoming continuous on the one hand with the quad- 

 rate yolk cells lining the alimentary canal below, on the 

 other with the cells bounding the great mass of yolk. This 

 continuity has been carefully represented in PI. IV, fig. 4. 

 Where the invagination cells cease would be difficult to state, 

 owing to the fact that the bending down at the sides is a 

 gradual process partly dependent upon the growth of the 

 mesoblast. 



The hypoblast can now be classed according to its develop- 

 ment under two heads, {a,) The cells above the alimentary 

 canal, which have arisen from invagination and are con- 

 tinuous with the reflected epiblast at the blastopore. This 

 we shall call the invagination hypoblast, [b.) Those cells 

 lining the alimentary canal below and those immediately 

 bounding the yolk elsewhere, which arise by histological 



