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J. F. GUDERNATSCH 



exceptionally occurs behind the last branchial arch. The single 

 parts of the basibranchiale define this ' thyreoid region' dorsally, 

 while ventrally the paired musculus sternohyoideus is spread out 

 beneath the organ. This region is at the same time that of the 

 ventral aorta and its branches to each of the gills. 



Thus the thyreoid gland is located along the trunk through which 

 the blood for the entire body is pumped from the heart into the 

 respiratory organs. The narrow cleft between the bony parts of 

 the floor of the pharynx dorsally and the muscles ventrally is 

 completely filled with thyreoid tissue except for the space occupied 

 by the large arterial trunks. This region, as we see from the 

 extensive literature on the visceral skeleton and musculature in 

 fish, in the manifold development of its bony (cartilaginous) and 

 muscular parts shows a decided tendency to vary. It is only 

 natural that this tendency should be found in the thyreoid gland 

 also; since it has to accommodate itself to the configuration of 

 the tissues just mentioned, a pronounced adaptability must be 

 of the greatest benefit to it. The property of variation is possessed 

 b}^ the thyreoid gland of the Teleosts to a most striking degree 

 and within the same species rather remarkable differences are found. 

 Twelve weak-fish (Cynoscion) for instance, all differed in the exten- 

 sion and position of their respective thyreoids. Similar conditions 

 were observed in other species. This variability within the species 

 may indicate that in the thyreoid gland we have a very unstable 

 organ, which perhaps in vertebrate phylogeny has not yet acquired 

 its final condition. We know that in the higher classes of verte- 

 brates there is the same variability among individuals regarding 

 the development of their thyreoid glands. In mammalia the 

 individual variation is very great. The lobes may have different 

 forms, and give to the organ a paired appearance, or there may 

 be a more or less well developed isthmus between them. Interest- 

 ing comparisons have been made especially in man.' In the 

 phylogenetically younger epithelial bodies the variation is still 

 larger. All of these facts indicate that the gill slits and their 



'^ Marshall ('95) examined the thyreoid glands in sixty children in which he found 

 all possible variations. 



