FURTHER STUDIES ON THE GERMINAL LAYERS ETC. 39 



. noticed in the longitudinal section. There becomes established later on 

 at this place the yolk plug, as reference to Fig. 22 will show, and 

 as has been proved sufficiently, I think, in the case of Trionyx 

 (loc. cit.). 



In Fig. 8, the section passes through the blastoporic passage 

 which is, however, still open above on the left. The floor of the 

 passage is proliferating cells below. 



In Fig. 9, tlirough the front part of the ventral opening of the blas- 

 toporic passage, the cells of the lower layer are arranged in the median 

 line in a regular columnar epithelium. This is what is usually known 

 as the chorda-hypoblast, although, as I shall show in the sequel, it gives 

 rise to something besides the chorda dorsalis. Laterally the chorda- 

 hypoblast becomes continuous with loose irregular reticulate masses of 

 lower-layer cells containing a large quantity of yolk granules. Still 

 more laterally the lowei' layer is reduced to a very thin sheet corres- 

 ponding to the transparent region of the surface view. Outside of 

 this it becomes continuous with the bed of yolk -matter. 



As we trace the series of sections forward, the part of the lower 

 layer showing a regular columnar arrangement becomes smaller aud 

 smaller, the loose netw(jrk-like part encroaching gradually on it. 

 Columnar cells remain longest on the lowest stratum of the median 

 part (Compare Trionyx, loc. cit., Figs 13 and 14). 



In Fig. 10, the whole lower layer hi the regicjn of the embryonic 

 shield consists of a loose network of cells. 



This embryo thus consists almost entirely of only two layers, the 

 epiblast and the primiti^ e hypoblast. The only part which we can 

 distinguish as the commencing mesoblast is a median mass of cells 

 found behind, and on the floor of, the blastoporic passage, continuous 

 above and below^ with the two primary layers. In front of the blas- 

 toporic passage, the chorda-hypoblast is being gradually arran<'ed in 



