566 



Couclusions. 



The thyroid gland arises as a median outgrowth from the floor 

 of the branchial cavity, in immediate contact with the anterior 

 extremity of the pericardium i. e. the coelom. It remains in con- 

 tiguity with the pericardium as long as its union with the floor of 

 the branchial cavity is retained. When the thyroid outgrowth is 

 severed from the floor of the branchial cavity, its posterior half divides 

 into two parts which migrate dorso-laterally, thereby losing connection 

 with the pericardium. At the same time some of the anterior cells 

 of the tyroid outgrowth are lost on the median part of the muscle 

 mylohyoideus. This muscle is formed from the fused dorsal and 

 ventral walls of an anterior prolongation of the coelom (fig. 5 and 6). 

 The union of cells from the thyroid outgrowth with this muscle 

 consequently further emphasizes the intimate connection primarily 

 existing between the gland and the wall of the body cavity. 



In a former paper *), I have called attention to a line of 

 ectodermic elevations on the trunk of the embryo serially continuous 

 with those that begin the branchial clefts, and extending towards 

 intersegmental widenings of the cavity of the alimentary canal, which 

 are presumably rudiments of true branchial pockets. The pronephric 

 Anlage lies ventral to this line of ectodermic elevations, and the ridge 

 which marks the union of the pronephric duct with the ectoderm is 

 continued anteriorly onto the head of the embryo at the ventral 

 margin of the branchial clefts. Boveri-) has demonstrated in Am- 

 phioxus an intimate connection between the nephric system and the 

 branchial clefts. It therefore seems not impossible that we have in 

 the thyroid and suprapericardial bodies modifications of organs be- 

 longing primarily to the pronephric system, and that the median 

 origin of the thyroid may be connected with the reduction of the hyo- 

 raandibular and oral clefts, which forced the associated nephric 

 organs to seek an independent opening to the exterior, which was 

 lost, together with their connection with the body- cavity, when the 

 function of the organs changed. 



The chief difterences in the development of the thyroid gland and 

 suprapericardial bodies in Nectur us and in Triton or Sired on, as 

 described by Maurer, are: 



1) Julia B. Platt, Ontogenetische Differenzirung des Ektoderms 

 in Necturus. Archiv f. mikroskop. Auatoniie, Bd, XLIII, 1894. 



2) Theodor Boveri, Die Nierenkanälchen des Aniphioxus, Zoologische 

 Jahrbücher, Abt. f. Anat. und Ontog. der Tiere, Bd. V, 1892. 



