516 The American Naturalist. - [June, 
ing to His’s discovery this is precisely what takes place, only the 
lips are brought together first at one end, where they at once 
unite, while behind they are widely separated; but gradually 
they are brought together and unite throughout their entire 
length. 
Concrescence is, then, a modified method of uniting the lips of 
a greatly elongated gastrula mouth. Why this modification is 
established we cannot say with certainty, though we may surmise 
with confidence that it is consequent upon the great accumulation 
of yolk in vertebrate ova. 
‘The view here adopted enables us to speak positively as to the 
point where we are to look in vertebrates for the homologue of 
the invertebrate mouth. In annelids concrescence is very well 
marked whenever the ova contain much yolk; now in leeches 
and earthworms, the ectental line does not concresce along the 
entire axial line but on the contrary, as shown by Kleinenberg 
and Whitman, the foremost part of the germ bands’ (gastrula lips) 
do not unite but leave a small opening; when the permanent 
mouth is formed this opening is carried in and serves as the pas- 
sage between the mouth cavity (Vorderdarm, stomodzeum) and 
the archenteric cavity. The foremost part of the line of concres- 
cence lies according to His’ observations on fishes just where the 
optic outgrowths arise; hence we have to search between the 
origins of the optic nerves for traces of the invertebrate mouth. 
(Zo be continued.) 
