14 ARTHUR DENDY. 



3. Systematic Account of thk Stages in Development. 

 Stage C^ (figs. 1 — 5). 



This is the earliest stage which I liave yet obtained, and to 

 it I assign embryos numbered 5, 6, 7, 8, 9. The eggs from 

 which these embryos were removed were collected on Stephens 

 Island on November 14th, 1897, and the embryos were re- 

 moved by me on November 22nd, the day on which I received 

 them. They were therefore at least eight days old. 



Already the blastoderm appears to have spread completely 

 around the yolk, the epiblast forming over the greater part of 

 its extent a thin transparent membrane, to the naked eye 

 resembling a vitelline membrane, but composed of a layer of 

 polygonal, nucleated, flattened, or in some parts columnar 

 cells. This blastoderm lay so near to the shell that I found it 

 impossible to open the egg without puncturing it and allowing 

 some of the very liquid yolk to escape. 



Beneath the superficial epiblastic layer of cells, and lying 

 in the outermost part of the yolk, are numerous nucleated 

 stellate cells which form a loose network holding the outer 

 part of the yolk together, and causing it to adhere to the under 

 surface of the epiblast, wliereby the blastoderm outside the 

 area pellucida acquires a thick, blanket-like character. These 

 cells beneath the superficial epiblast may be termed " lower- 

 layer '' cells (fig. 4, L.L.Y.). 



Over a certain area, corresponding approximately to the 

 area pellucida, a large cavity has made its appearance in the 

 lower layer ; this is the segmentation cavity (fig. 4, S. C), 

 whereby the true embryonic portion of the blastoderm is widely 

 separated from the underlying yolk. The floor of the segmen- 

 tation cavity is formed by a very thin and delicate membrane, 

 which I propose to call the sub-embryonal membrane (fig. 2, 

 S.E.M., and fig. 4, the dotted line). The sub-embryonal 

 membrane is composed of yolk-spheres held together by a 



' I commence with the letter C in order to make allowance for earlier 

 stages which may be forthcoming later on. 



