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and slightly anterior to, the anterior edge of the mass of cells that, 

 at this age, represents the hyomandibulo-symplectic cartilage of Parker's 

 nomenclature. The cleft might therefore be taken to lie morphologically 

 anterior to that cartilage. Fenestra 2 of Parker's nomenclature would 

 then represent the still unclosed space between the hyoid and man- 

 dibular arches, and the hyomandibulo-symplectic would belong to the hyoid 

 arch. If, on the contrary, the position of the cleft be taken to indicate 

 that it has been occluded by a backward translation of the hyomandibulo- 

 symplectic of the fish, this latter cartilage would lie morphologically 

 anterior to the cleft, and would belong to the mandibular arch. My 

 embryos do not permit of a definite choice between these two sup- 

 positions. 



The cleft next posterior to the hyomandibular one arises, in my 

 embryo, from the dorsal edge of the pharynx, as v. Kuppfer states, 

 and it is the first cleft posterior to the velar folds. It is thus cer- 

 tainly v. Kupffer's second cleft; that is, the hyobranchial. It runs 

 backward and outward, internal and hence posterior to the ceratohyal 

 of Parker's descriptions, and ends blindly, posterior to that structure 

 and at some distance beneath the outer surface. The next posterior 

 cleft ends blindly in the same manner as the hyobranchial. Whether 

 these two clefts ultimately entirely abort, or later break through on 

 the outer surface, makes no difference whatever in the evident deduction 

 that the ceratohyal of Parker's descriptions is certainly not a first 

 branchial arch, as Ayers and Jackson conclude that it is. Posterior 

 to this arch the sections were so imperfect that I could not determine 

 the relations of either of the clefts to Parker's first branchial arch. 

 This latter arch may, however, be, as already stated, a part of the 

 hyomandibular, its backward translation thus accounting for the closing 

 of the external openings of the hyobranchial and next posterior clefts. 



The presence of these two partially aborted clefts, posterior to. 

 the hyomandibular cleft, in this embryo, thus accords with the de- 

 duction made from the comparison of the total number of branchial 

 pockets in v. Kupffer's somewhat younger embryo, with the total 

 number of clefts in the adult. 



In my badly preserved specimen the first branch of the vagus 

 that I can recognize goes to what I take to be the constrictor oesophagei 

 muscles. Whether there are, in the embryo, two of these muscles, as 

 there are in the adult, or only one, I could not tell, neither could I 

 tell certainly whether there was but one nerve here or two. The next 

 branch of the nerve is large, and forks over the 4th cleft ; that being 

 the first cleft that opens on the outer surface. The two constrictor oeso- 



