84 Geometrical relation of Nuclei 



extrinsic factors, in this case chiefly food yolk granules which are 

 distributed in larger or smaller grains in greater or lesser quantities 

 within the meshes of the living protoplasm in the different regions 

 of the egg. 



In the paper alluded to before, I have attempted to show 

 how the process may work in an Amphibian egg, but a mathe- 

 matical demonstration alone would prove or disprove its correct- 

 ness. 



In the developing egg of Rana tempomria we see a gradual 

 conversion of large segments into small segments, there being 

 always a rather sudden transition from the smaller, usually darker 

 cells, into the larger, usually less pigmented cells ; this not very 

 well denned transition area forms what we might call a differ entia- 

 tion zone gradually passing from the upper towards the lower 

 pole. There is no migration of cells, it is merely a wave like 

 progression of a zone of differentiation. After passing the equator 

 one arc of the zone instead of progressing over the surface dips 

 inwards, giving rise to a deep groove and then a cleft the first 

 sign of the blastopore. 



A section taken vertically through the egg and this groove 

 shows the same rather sudden mergence of the smaller dark cells 

 into the larger light cells at the bottom of this groove just as it 

 appears on the surface at the opposite side of the zone. 



The part of the zone still on the surface sweeps on but adjoining 

 the spot where the groove is, more and more of it turns inwards 

 until at last the whole zone of mergence disappears from the surface 

 but may be found beneath, where it is probably still extending 

 and giving rise by differentiation to gut lining cells and causing 

 a cavity by a split, which widens later into the true archenteron. 

 This departure inwards which does not involve any movement of 

 cells is shown in the accompanying figure (Fig. 39), and con- 

 stitutes protogenesis, or as I called it in the paper alluded to, the 

 result of the primary centre of cell activity. The process is some- 

 what masked by another phenomenon, the growth backwards of 

 the rim of the blastopore Bl as soon as and wherever formed, which 

 ultimately produces the growth in length of the animal, and meta- 

 merically segmented region, i.e. deuterogenesis (v. arrow in 

 drawing). The ventral part dies out as an active proliferation 



