ON THE PROCESS OF GASTRULATION IN CHELONIA g 33 



(h) the lower layer (the cœnogenetic hypoblast of Hubrecht, 

 the parader m of Knpffer, and of Mehnert, the lecitophor of Yun 

 Beneden.) The superficial layer, which T shall call the epi blast, 

 forms a distinct membrane and is composed of columnar cells in the 

 region of the embryonic shield, but changes gradually into low 

 cells in the parts outside the shield. The lower layer is composed of 

 irregular, amoeboid-shaped cells and does not probably form a con- 

 tinuous membrane. I hope to show in the sequel that this layer 

 ought to be regarded as only a part of the hypoblast, and might be 

 called the cœnogenetic hypoblast, after Hubrecht (No. 5 ) . In 

 the region of the primitive plate, there is a different state of 

 things. Instead of having two distinct layers, this area shows a thick 

 accumulation of cells. It is composed for the most part of an 

 irregular network of cells with tolerably wide meshes between, so that 

 it is not a compact mass. In the middle of this accumulation, there 

 is seen the i n v a o- i n a t i o n - c a v i t y — the a r c h e n t e r o n — leadino' at 

 first downwards but soon forwards and ending blindly. The roof of 

 this cavity shows distinctly a columnar arrangement of cells, and 

 becomes continuous with the epiblast at the anterior lip of the blasto- 

 pore. On the floor, as well as for some distance behind the blasto- 

 pore, so long as we are in the region of the primitive plate, 

 we see no columnar arrangement : the general network of the mass 

 extends up to the surface. There is no sharp line of demarcation 

 between the cell ii lar mass of the \) v i m i t i v e plate a nd the subjacent 

 bed of the yolk. The latter is divided into especially fine globules at 

 the boimdary line, and we can clearly see many cells arising at this 

 ]»]ace and adding themselves to the primitive knob. That the 

 nuclei of these cells are the descendants of the segmentation nucleus, 

 there can be no reasonable doubt ; in fact I would have this 

 addition of new cells considered simply as the continuation of the process 



