FURTHER STUDIES ON THE GERMINAL LAYERS ETC. 37 



"a study of Fig«. 5 and 5a (PI. Ill, a longitudinal section) and 

 Figs. 6-10 (a series of transverse sections). 



Fig. 5 is a longitudinal section of this stage, near the median line. 

 The blastoporic passage is very conspicuous, beginning at the dorsal 

 surface and leading obliquely forwards to the ventral surface. There 

 is a marked difference of structure between the part of the blastoderm 

 in front of the passage and that lying b e h i n d it. In front of the 

 passage, there are only two layers : the epiblast and the primitive hypo- 

 blast. The epiblast is already a well defined sheet, extending beyond 

 the area included in the figure — though how much beyond I have not 

 ascertained. In the transparent area in front, it is a thin layer con- 

 sisting of pavement cells ; as we trace it posteriorly however, the cells 

 become taller, changing gradually from the pavement to the columnar 

 shape. In the region directly in front of the dorsal opening of the blas- 

 toporic passage, the nuclei are in several strata, although the epiblast 

 seems to consist in reality of only a single layer oftall cells. As regards 

 the lower layer or primitive hypoblast, it is, in the anterior transparent 

 area, a loose layer of cells containing many fine yolk-granules (Com- 

 pare Trionyx l\o. 1, Fig. 15). In front it passes into the bed of yolk 

 in which nuclei are seen. Traced backwards, it becomes at first only 

 two or three cells deep, and then suddenly quite thick as we reach the 

 outer of the two concentric zones seen in the surface view of the em- 

 bryonic shield. The cells here are arranged in a loose network, with 

 large meshes. Further backwards, the lower layer is formed of 

 columnar cells and assumes an appearance like the superjacent epiblast. 

 It is thicker, however, and seems to l)e formed of severid layers of cells. 

 At the anterior dorsnl lip of the l^lastoporic passage, the epiblast 

 becomes continuous with the primitive hypoblast. 

 ^ In that part of Fig. 5 (or Fig. 5a) showing the region behind 

 the blastoporic passage, there is a different state of things. Beginning 



