492 
J. GRAHA!^[ KERR. 
tion of the intestine may be looked ou as a repetition of a 
phylog’enetic stage,' in whicli the intestine had assumed a 
spiral coiling owing to its relatively great length in com- 
parison with the length of the splanchnocoele. As was shown 
especially by Riickert ^ the spiral valve in certain Elasmo- 
branchs is similarly preceded by a spirally coiled condition 
of the endodermal gut rudiment, in this case definitely asso- 
ciated with actual growth in its length. As, further, the 
spiral valve is characteristic of all the more primitive groups 
of fish-like Guathostomata, Elasmobranchs, Crossopterygians, 
and Lung-fishes, not to mention Actinopterygians and ancient 
groups of amphibians and reptiles, we may take it as fairly 
piobable that the Gnathostomata were as a whole character- 
ised during an early period of their evolution, a period ante- 
cedent to the splitting up into the groups above named, by 
the possession of a long spirally coiled gut.'”' 
In comparing the stage under discussion (XXXVI) with 
the preceding stages it will be seen that the using up of the 
yolk is still taking place most actively in the anterior region. 
As a consequence it is seen that the first turn of the spiral 
has become greatly reduced in size, so tliat, as shown in 
ventral view, it is decidedly smaller than the succeeding 
turn instead of being much larger, as was the case in 
stage XXXV. Now that the turns of the spiral are distinct 
it may be seen that there are in all most usually nine or ten 
turns. 
In the last stage figured (stage XXXVII, text-figs. 3 D and 
4 d) the yolk has been used up to such an extent as no longer 
to influence the outward form of the intestine. The turns of 
the spiral are now of approximately uniform diameter, they 
are closely bound together by mesenchyme, and are enclosed 
‘ Graham Kerr, ‘Phil. Trans. Roy. Soc.,’ B. cxcii, 1900, p. 325. 
- ‘ Arch. f. Entwick. Mech.,’ iv. 1890, p. 298. 
^ In all probability the gut has varied much in length from time to 
time during the evolution of the vertehrata. changes in length being 
associated with changes in the nature of the diet, e.g. from vegetable to 
animal. 
