348 Thos. H. Montgomery. 



first movement of the caudal lobe. Balfour (1880) referred to 

 "elongation of the dorsal region, i. e., the region on the dorsal sur- 

 face between the anal and procephalic lobes" ; but he failed to account 

 for such dorsal elongation. Locy (1886) held that there is a shorten- 

 ing of the ventral surface of the embryo which draws the tail-lobe 

 forward, thus explaining how the apex of this lobe becomes separated 

 from the surface of the embryo; '^a shortening of the ventral band 

 does take place, which is at least equal to the pre-existing tail-fold, and 

 the tail is in consequence drawn forward ventrally." The cause of 

 "the ventral movement of the passive yolk mass must be found in the 

 relative pressures exerted upon it by the dorsal area, on the one hand, 

 and the ventral area, on the other. * * * Xhe principal force, 

 then, that pulls the nerve bands away from the ventral surface is the 

 one which tends to reduce the width (not as Balfour will, the one 

 that increases the length) of the dorsal region. The evident cause 

 for this reduction in width is the dorsal concentration of the ecto- 

 dermic elements which accompanies the formation of the so-called 

 terga, and this is also the cause for the descent of the yolk mass." 

 Wallstabe has given the most detailed description of the external 

 changes during reversion. 



Each investigator who has tried to explain the mechanics of rever- 

 sion has differed from his predecessor, and so must I. The process is 

 a very complex one and can hardly be ascribed to any single factor. 

 The first intimation of the process is the widening of the ventral 

 sulcus, particularly in its middle portion (compare Fig. Y4, Plate 

 VI, with Fig. 78, Plate VII). This widening cannot be due to auto- 

 matic growth of the ventral sulcus itself, for this is composed of a 

 very thin plate of cells hardly capable of exerting a strong pressure 

 upon the thicker and heavier regions of the embryo; this widening 

 of the ventral sulcus is rather a result than a cause of the reversion. 

 A pressure is exerted upon the yolk by the invagination of portion of 

 the cephalic region (stomodaeum, ganglia, Figs. 89, 91a) ; this pres- 

 sure would be transmitted through the yolk to the weak portion of 

 the embryo, the ventral sulcus, and helj) to widen the latter. But 

 such pressure alone would be incapable of producing the whole set of 

 changes that take place. A more powerful factor is, probably, in- 



