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Nachdruck verboten. 



The Spiraciilar Grill Cleft in Lepidosireii and Protopterus. 



By W. E. Agae, B.A., 

 Junior Assistant in Zoology at Glasgow University. 



With 5 Figures. 



In the course of an investigation into the development of the 

 skull of Lepidosiren paradoxa, the material for which was kindly 

 placed at my disposal by Professor J. Graham Kerr, a peculiar tu- 

 bular organ was noticed, enclosed in a hoop of cartilage springing 

 from the lateral wall of the auditory capsule. This organ was soon 

 identified with an organ described by Pinkus in Protopterus (Die 

 Hirnnerven des Protopterus annectens, Morphol. Arbeiten, Bd. IV). 

 In this paper Pinkus remarks: "Das Organ ist zweifellos ein Derivat 

 des Seitenkanales", but in the absence of comparative or developmental 

 material refrains from making further suggestions as to its significance. 

 As it seems to be an organ of unique character, I propose to speak 

 of it as "Pinkus' Organ". From the complete series of developmental 

 stages of Lepidosiren collected by Professor Kerr I have been able 

 to determine that it is not part of the lateral line system of sense 

 organs, but is directly formed from the epiblastic "invagination" of 

 the spiracular cleft. 



Fig. 1 shows a section through the rudiment of the spiracle at 

 Kerr's stage 31 (thirteen days after hatching). The division between 

 the epiblast and hypoblast is very sharp owing to the presence of 

 large yolk granules in the latter. The whole rudiment of the pharynx 

 and gill clefts at this stage is solid. 



A few sections further forwards the hypoblast reaches up to the 

 skin ; the epiblastic ingrowth is therefore not so wide autero-posteriorly 

 as the hypoblastic outgrowth. The hyoideo-mandibular branch of the 

 Vllth nerve is shown, a branch of it running to the rudiment of a 

 lateral line sense organ. It runs, of course, behind the spiracular 

 rudiment. This nerve runs very close to Pinkus' Organ through all 

 the early stages of development, curving round its posterior surface 

 so closely in contact that it is impossible to say from the appearance 



