No. 2.] FORMATION OF THE FISH EMBRYO. 433 



pointing to the loss of some cells from the germ-ring, and to 

 their conversion into cells of the extra-embryonic region. 



There is direct evidence to show that the germ-ring, after 

 it has passed the equator, is entirely inadequate to contribute 

 any large amount of material to the embryo. The sides of the 

 embryo are then far too great, in proportion to the germ-ring, to 

 allow any such formation as His supposed. It must be remem- 

 bered, too, that in these pelagic fish eggs, the embryo elongates 

 posteriorly pari passu with the overgrowth of the germ-ring. 

 We have seen that during overgrowth both upper and under 

 layers of the germ-ring have decreased in quantity, and in 

 the number of their cells. We can account for a part 

 of this loss in the uppermost layer, by the continuous addi- 

 tion it seems to make to the extra-embryonic region. We can- 

 not account in a similar way for the loss of the cells of the 

 under layer (meso-endoderm). We have seen that there is 

 always a larger accumulation of these under-layer cells near to 

 the embryo, and the number of cells is less and less as we pass 

 outwards along the germ-ring. This strongly suggests that the 

 cells are passing continuously from the germ-ring into the 

 embryo. This must certainly be the case after the equator 

 of the ^^g has been passed by the germ-ring, and sections 

 show that it is in these latter stages of overgrowth that the 

 contrast between the different portions of the germ-ring is 

 greatest. 



Whether or not the same thing happens before the equator 

 is reached, is not so easily determined, because during this 

 period the germ-ring itself is getting longer, and the increased 

 length might account for the decrease in the number of cells of 

 the inner layer in each cross-section. But during these early 

 stages the sides of the embryo are not so sharply marked off 

 from the germ-ring, and there is direct evidence to show that 

 this lateral material passes into the embryo along each side of 

 the middle line, while the tail-knob grows backwards along the 

 axial mid-line. 



I therefore conclude that during the elongation of the em- 

 bryo, material is continuously passing in from the germ-ring, 

 and is laid down with the axial material already present. 



