733 



Tlie presumed hoinologv of the thymus and branchionepliros has 

 also been supjioited from the side of the craniates, now iJiat, in 

 the development of such a primitive form as Heplanciins, the presence 

 in liie thymus of excretory dncis, whicii in Amphioxns analogonsly 

 open into the branchial clefts, has been shown. 



If the brancinonephros develops from the branchial epilheiinm. 

 the chief ditTicnlty to homoiogize it with the thj iiius, 1 thitdi then 

 lies in the period of development of tliis gland. One should expect 

 the thymus to become perceptable in a very early period of its 

 development, but this only happens very late. 



The reason for this is because the original function no longer comes 

 to development. It is taken over by the pronephros and the meso- 

 nephros. The other funclion of the thymus i.e. its inleinal secretion, 

 caused by the lyinphocytlike cells, must phylogenetically have origin- 

 ated much later. 



2. Spiraculnr sense organ. 



\\\ no vertebi-ates does a division of the thymus come to dexelop- 

 ment in the first ()ranchial cleft (spiracle). It appears not even to 

 be formed there al all. On ihe other hand, we find on the wall 

 of the spiracle in the embryos or larvae of Ihe moi-e primitive 

 fishes: Selachians, Ganoids and r)i|)n()i a sense organ, which is not 

 met with on any of the remaining branchial clefts. These adult fishes 

 also possess one. 



We find it even in those forms (Dipnoi and Holostei) in which 

 the spiracle, which is developed in the manner of an intestinal 

 pouch, no longer breaks through outwardly. 



It was discovered by Ramsay Whight in 1885, who found it as 

 a protrusion of the medial wall of the spiracular visceral pouch of 

 the Holostei (Lepidosteus and Amia). This protrusion (diverticulum) 

 is directed upwards and surrounded by the cartilaginous auditory 

 capsule; in other words, it lies in a canal of the lateral cartila- 

 ginous wall of the otic region of the skull, but otherwise has no 

 relation to the auditory organ. 



A similar canal in the cranial cartilage, into which adiverticulum 

 of the spiracular wall penetrates, was discovered by Bridge in 

 Polyodon. The same was also observed by Wkight in the sturgeon. 

 The presence of a sense organ in these Chondrostei is. however, 

 not mentioned. 



Wiitr.HT found, that in the Holostei this sense organ is innerxated 

 by a hi'anch of ihe ram. oliciis of the Incial nerve, which in ihc 



