MOUTH. 



28l 



by a new mouth derived from a fusion across the mid- 

 ventral line of a pair of gill-clefts. DOHRN was a trifle 

 uncertain as to the rudiment of the old mouth, but BEARD 

 was more certain on this point, and thought he had estab- 

 lished the fact that the hy- 

 pophysis cerebri represented 

 the remains of the old An- 

 nelid mouth. 



Dohrn certainly succeeded 

 in bringing forward some 

 apparently good evidence in 

 support of his theory of the 

 gill-slit origin of the mouth. 

 This evidence was derived 

 from the study of the de- 

 velopment of the mouth in 



Teleostean or bony fishes. Fig . I33 ._ TWO frontal views of an 



In manv Teleosteans the ernbrvo of Batrachus tan, to show the 



double nature of the stomodoeum. (From 



mouth has at first an appar- hitherto unpublished drawings kindly lent 

 , , , , , ... by Miss C. M. CLAPP.) 



ently double origin, in that The embrvo is lying upon the yolk 

 two separate ectodermal in- and the se P tum which divides the stomo- 



doeum passes from the upper lip to the 



growths OCCUr which fuse surface of the blastoderm which covers 



, i , i j j i the yolk. The lower figure is a drawing 



With the endoderm, instead O f th e sa me embryo as the -upper, a few 



of the median stomodoeal hours later - Above the stomodceum are 



seen the small nasal pits (rudiments of 



involution which is SO char- the external nares), and at the sides of 

 , r .1 -IT the head are the rudiments of the eves. 



actenstic of other Verte- 

 brates. This double origin of the mouth is particularly 

 well shown in the embryos of the remarkable toad-fish, 

 Batrachus tau, as observed by Miss CORNELIA CLAPP at 

 the Marine Biological Laboratory of Woods Roll, Mass., 

 in 1 889 (Fig. 133). In this case the mouth-cavity is seen 

 to be divided into two halves by a median septum. 



Subsequently the septum becomes absorbed, and the 



