PLATE XLII. 



Fig. 1. P.ohlongus. Sections drawn at intervals. Young ring .stage cut transversely into sixty- 



(94) three sections (.0075 mm.). [6c = blastocoele; ep = epidermis; p6 = periblast). 



Diagram 12 = longitudinal median section, constructed from the transverse sections. .0075 



mm. X280 equals apparent thickness of each section. This amounts to 2.1 mm. 2.1 X 63 



gives apparent length of the cap, which is 132.3 mm. Measuring off 132.3 mm., I divided it 



^ into sixty-three equal parts, and then by vertical measurements of each section constructed 



the diagram. Thickness of upper layer in the sections. (Apparent of magnified 280). 



The second section takes in many epiblast cells, which are seen partly in surface owing to the 

 convexity of the cap near margin. 



Nearly all the cells are in Home atage of diinsion throughout all the sections. 



Notice that the nuclei of the periblast are few in section two, more in three, most in five and 

 are fewer towards the middle of cap (37th). They are more numerous beneath the embryonic 

 fold than elsewhere under the ring. 



The lower layer becomes distinct only in section five, where we see not only a line of division, 

 l)nt notice that the nuclei retreat from this line somewhat. 



The lower layer is thickest (3-4 cells deep) in the embryonic fold; elsewhere the ring where 

 it is well begun is 1-2 cells deep. The ring is seen as a true infolding in the thirty-second 

 section and especially in the thirty-fifth. In some points the ring is scarcely begun, i. e. there 

 is no distinct ring. The axial portion of the ring ceases with the tenth section, being thus less 

 than ^ of the entire length of the caj). 



The upper layer is thickest over the embryonic apical fold, being here from 4-6-ce!ls deep. 

 This thick area thins off grad\ially beyond the twenty-fifth section {i. e. between twenty-fifth 

 and thirty-fifth). From the thirty-fifth onward the diminution in thickening continues but 

 in a much less marked degree, dwindling down to about half the thickness of the embryonic 

 fold. 



The thickness of the upper layer is nearly uniform in all transverse sections from margin to 

 margin. 



The upper layer is composed of an epidermis and a deeper neural layer (neural at least in its 

 apical region). The epiblast is composed of flattened cells that abut against the highest angle 

 of the periblast. In no case does the ring appear to arise by an infolding of this laj'er; on the 

 contrary, there is sometimes a small space left between the marginal epiblast cell and the nearest 

 cell of the ring (so in 27th). The deeper neural layer, however, bends into the infolding lower 

 layer as .shown in the twenty-seventh, thirty-second, thirty-fifth, and thirty-seventh. The 

 nuclei may be seen in all stages of division in the ring region. 



The number of cells in a section of the ring varies from two (32nd) to four (27th, 37th) . In the 

 floor, the ring is one cell deep (except in apical region — see above) or sometimes two cells deep 

 (most often at the inner edge). Later it is plainly one cell deep. 



