GEOWTH IN LENGTH OF THE FEOG BMBETO. 243 



have closed more rapidly than the dorsal. Dotted line and arrows indicate 

 same features as in Fig. 9. 



Fig. 11. — Frog embryo when the neural plate is a conspicuous object ex- 

 ternally, and is deeply grooved. The embryo has become very distinctly 

 elongated, owing to the growth due to the secondary area of proliferation. 

 This was drawn with a camera. The dotted line and arrows represent my 

 own interpretation of the facts as in the preceding figures. 



Fig. 12. — A section of the same stage as Fig. 8, semi-diagrammatic. The 

 dotted line indicates the location of the split amongst the cells, whereby the 

 archenteron is supposed to have originated and become prolonged forwards. 



Fig. 13. — A diagram of a stage intermediate between Figs. 9 and 10. The 

 portion of the gut cavity indicated by the dotted line represents that part 

 which I suppose to be formed by the splitting amidst the yolk-cells. The 

 future continuation of this slit is represented by a prolongation ventral- 

 wards of the dotted line. The posterior part of the roof of the gut cavity, 

 marked with small dots, is that part formed by the active growth of the 

 dorsal lip of the blastopore, which is shown by the diagonal shading. 



FiQ. 14. — A diagram of a later stage, such as Fig. 11. In this the seg- 

 mentation cavity has become entirely obliterated. The secondary area of 

 proliferation is completed and active all round the blastopore. 



Fig. 15. — A diagram to show the sequence of the horizontal furrows during 

 segmentation of the frog's egg. 



Fig. 16. — A diagram to show in which direction the process of segmentation 

 will incur least resistance, on the supposition that the yolk is distributed as 

 indicated by the intensity of the shading. 



Figs. 17 — 20. — Figures of the frog's egg during the formation of 

 the blastopore, to show which part of tlie surface of the ovum forms the 

 yolk-plug. Each figure is arranged in the same position. 



Fig. 21. — A section of a frog's egg of a stage intermediate between Figs. 

 8 and 9. The ovum was drawn with camera. 



