1881.] Structure and Development of the Skull in Sturgeons. 143 



We have thus been breaking ground, rather actively, among this 

 most important and almost extinct "order" of fishes; a very im- 

 portant addition to the work will soon come to hand, namely, a 

 memoir by Professor Traquair, on the adult skull of Lepidosteus. 



I mention these details to show that biologists are working with a 

 will, and working as yoke-fellows ; and no doubt remains with me as 

 to the result of snch co-operation. 



I must refer the reader to Professor Salensky's invaluable work on 

 the development of the sterlet (Kasan, 1878), unfortunately published 

 in Russian, and to the second volume of Mr. Balfour's work, for an 

 account of the earliest stages of the Acipenserine embryo. 



Even in larvae one-third of an inch in length, the cartilage was 

 becoming consolidated, and I was able to work out, by sections and 

 dissections, the structure of the cranium and visceral arches ; the 

 one specimen which was seven- twelfths of an inch in length, and which 

 was made into a large number of extremely thin sections, left nothing 

 to be desired. 



As I hope for an early publication of the present paper, I shall not 

 enter very largely into details ; they can only be followed, profitably, 

 by help of the numerous illustrations. 



The development of the skull of the sturgeon is very similar to 

 what we find in the sharks and skates (" Selachians "), but the sus- 

 pension of both the mandibular and the hyoid arches by one pier, 

 derived from the hyoid (the hyostylic skull), which is seen in the 

 Selachians on one hand and on the Holostean Ganoid and Teleosteans 

 on the other, attains its fullest development in the " Acipenseridse," 

 or Chondrosteous Ganoids, for in them the " symplectic " is a 

 separate cartilage, and not a mere osseous centre as in Lepidosteus and 

 the Teleostei. 



Here I find a very noticeable fact, namely, that whilst in the 

 salmon the metamorphosis of the simple primary arches of the face 

 can be followed step by step, in the sturgeon the peculiar modification 

 of the arches shows itself during chondrification ; the hyoid arch, from 

 the first, is inordinately large. 



Notwithstanding the huge size of the subdivided hyoid pier, its 

 head only articulates in the larva with the auditory capsule ; later on 

 the basal cartilage reaches it, as in the Selachians. 



But the arches that retain their normal size lend no colour to the 

 theory that the visceral arches are related by their dorsal ends to the 

 paired cartilages that invest the notochord, a state of things like that 

 seen in the ribs and in the superficial cartilaginous hoops that sur- 

 round the huge pharynx of the lamprey. 



Mr. Balfour has demonstratively shown that in the branchial region, 

 when the pleuro-peritoneal cavity has been subdivided by the hypo- 

 bias tic outgrowths of the pharynx, the aortic arches lie inside the 



