MOUTH. 



281 



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. pig. 133.- Two frontal views of an 



In many TeleOSteanS the en^bno of Batradms ta„. to show the 



-^ double nature of the stomodceum. (From 



mouth has at first an appar- hitherto unpublished drawings kindly lent 



. , , , • • ■ ...1 ^ bv Miss C. M. Clapp.) 



ently double Origm, m that ' ^he embryo is lying upon the yolk, 



two separate ectodermal in- ^"^ "i<= ^'=p«"™ "h'* divides the stomo- 



dffium passes from the upper hp to the 



growths occur which fuse surface of the blastoderm which covers 



■ ,i ,, ,1 - . 1 the volk. The lower figure is a drawing 



With the endoderm, mstead „f .^^ ^^^^ ^^^r^^ as the upper, a few 



of the median StOmodceal ho"" 'ater. Above the stomodo?um are 



seen the small nasal pits (rudiments of 

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

 . . -- ^, -i-r . the head are the rudiments of the eves. 



acteristic of other verte- 

 brates. This double origin of the mouth is particularly 

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

 Batrachus tan, as observed by Miss Cornelia Clapp at 

 the Marine Biological Laboratory of Woods Holl, Mass., 

 in 1889 (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 



