42 OBSERVATIONS ON THE DEVELOPMENT OF RAIA BATIS 



divided into an outer and inner portion by a membranous septum ; the former being 

 the external auditory canal, and the latter the Eustachian tube and the cavity of the 

 tympanum. It will thus be seen that the spiracle is not only a true branchial fissure 

 in the first place, but that in the end it is homologous with the Eustachian tube and 

 the outer auditory passage before these are separated from each other by the membrane 

 of the tympanum. 



Professor Huxley, in a series of lectures * on the Vertebrate Skeleton, in which the 

 homologies and development of it are discussed with great ability, sets forth a some- 

 what different view with regard to the formation of the external ear, and maintains that 

 the first step is similar to that in the case of the eyes and nose, viz. an " involution " 

 or a " pushing in " of the integument. Professor Huxley's observations were made on 

 the chick, and he arrives at the same conclusions as Remak, leaving us to infer that the 

 auditory passage and Eustachian tube have no connection with the branchial fissures. 

 We have gone over the same ground in the pig, and have found Reichert's observations, 

 as mentioned above, fully confirmed. 



The relation of the spiracle to the branchial fissures is still further shown by the fact 

 that in some species, as in Scyllium and Laemargus, it, like the others, is provided with 

 respiratory fringes. In the skate this is not the case, but in the adult a comb-like fold, 

 resembling, and probably having the functions of, a gill, is found just within the spi- 

 racular opening. - 



The following is a general summary of the results contained in the preceding 

 pages. 



1. The yelk case is formed in the glandular portion of the oviduct, and is begun pre- 

 viously to the detachment from the ovary of the yelk which is to occupy it. 



2. The embryo, before assuming its adult form, is at first eel-shaped, and then shark- 

 shaped. 



3. The embryo is for a short time connected with the yelk by means of a slender 

 umbilical cord ; the cord afterwards shortens, and the young skate remains in contact 

 with the yelk until the end of incubation. 



4. There are seven branchial fissures at first ; the foremost of these is converted into 

 the spiracle, which is the homologue of the Eustachian tube and the outer ear canaH 

 the seventh is wholly closed up, and no trace remains ; the others remain permanently 

 open. 



* Structure and Development of the Vertebrate Skeleton, London Lancet, July, 1863, p. 427, American 



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