333 



one that would correspond exactly to an adductor hyomandibularis, 

 and an outer one that might become, if it persisted, a hyohyoideus. 



Turning now to the visceral clefts, v. Kupffer says (17, p. 29), 

 of the hyomandibular cleft; "eine Eröffnung dieser Tasche findet später 

 nicht statt". Nothing definite is said, in this respect, as to the other 

 branchial pockets, which might lead one to assume that they all break 

 through to the outer surface and persist as permanent clefts. This 

 assumption is, moreover, in full accord with Dean's statement (7, p. 270) 

 that "there is no evidence whatever of a degeneration of a series 

 of foremost gill clefts": but it is not in full accord with a footnote 

 on the same page, in which Dean speaks of a "doubtful cleft which 

 is early suppressed; this lies close behind the hyomandibular and 

 would, therefore, correspond to the thyroidean cleft of Dorn". 



v. Kupffer says, on p. 69 of the work just above quoted, that 

 in his embryo IX there were 15 pairs of branchial pockets, the hyo- 

 mandibular pocket being considered as the first or most anterior one. 

 In the adult, thirteen permanent clefts, making fourteen with the closed 

 spiracular cleft, is the maximum number given by Ayers and Jackson, 

 the average total being thirteen. It might therefore be inferred, by 

 comparison with embryos, that not only the hyomandibular cleft, but 

 also two other clefts usually aborted. And that the two clefts that 

 may so abort must in all probability be anterior and not posterior 

 clefts is seen, in addition to the reasons already given by Max Für- 

 bringer (8), by the following comparison of B. dombeyi with B. hetero- 

 trema. Müller shows the first lateral mucous gland lying opposite 

 the sixth muscle segment (18, Fig. 3, PI. 6). I find it lying opposite 

 the same segment in the one adult B. dombeyi that I have examined. 

 Müller shows the first branchial cleft of B. heterotrema between the 

 21st and 22nd mucous glands. In the one adult B. dombeyi above 

 referred to I find the first cleft between the 14th and 15th glands; 

 that is, just 7 glands anterior to the first cleft in B. heterotrema. 

 Adding this 7 to 6, the number of existing clefts in B. heterotrema, 

 gives 13, a number representing the number of clefts sometimes found 

 in B. dombeyi. 



In my badly preserved 12 mm embryo, which is evidently some- 

 what older than v. Kupffer's embryo IX, the first cleft that opens 

 on the outer surface is what I take to be the 4th cleft of v. Kupffer's 

 numbering. The hyomandibular cleft, which arises from the ventral 

 portion of the pharynx, as v. Kupffer states for younger embryos, 

 runs outward and backward and ends blindly immediately internal to, 



