Manchester Memoirs, Voi. I. {igo6), No. ^. 15 



not passive, since they move into the segmentation cavity, 

 are certainly not in a state of active division, and I think 

 we may say quite confidently that the roof of the archen- 

 teron does not come from this source. It consists of cells a 

 little larger than the cells of the animal hemisphere, but 

 resembling the latter closely in being pigmented and in 

 containing few yolk-granules of small size. This, however, 

 is by itself no adequate proof that that is their origin'' ; on 

 the contrary, a careful study of all the stages seems to 

 show that it is the rapidly dividing cells of the subequa- 

 torial intermediate zone which principally contributes 

 to the inner sheet of the overgrowth; not, perhaps, entirely 

 though, for inner and outer sheet are continuous at the 

 blastoporic lip, and many of the cells of the latter are 

 probably being perpetually rolled over into the former ; 

 the radiate arrangement of the cells seems a sufficient 

 demonstration of this. The outer ectodermal portion of 

 the fold is very largely derived from an extension of the 

 roof of the segmentation cavity ; the cells here are 

 certainly in a state of active proliferation, and as the total 

 thickness of the layer does not increase, the total area must 

 necessarily enlarge. This spreading of the animal cells 

 seems indeed to be directly concerned (mechanically) in 

 the first formation of the fold at the blastoporic lip ; for 

 if (as, for example, by growing the eggs in certain 

 solutions which affect primarily the yolk-cells and prevent 

 their movement into the segmentation cavity) the closure 

 of the blastopore is experimentally delayed, either the 

 roof of the segmentation cavity at once becomes thrown 

 into numerous wrinkles or the number of cell-layers 

 present in it is increased. 



^ It may be pointed out in this connection that Samassa ( " Studien liber 

 den Einfluss des Dotters auf die Gastrulation II.", Arch. Ent.-mech. vol.2, 

 p. 370 — 393, 1896) has shown that if in the eight-celled stage the four vege- 

 tative cells are killed, a typical blastoporic lip and archenteric cavity are 

 subsequently developed from the animal cells alone. 



