748 J. F. GUDERNATSCH 



Near the second branchial arteries the skeletal arch becomes 

 flattened again, the copula does not reach so far down, and first 

 the dorso-ventral and later the lateral distribution of the folli- 

 cles again increases. Comparatively few follicles now appear 

 below the aorta (fig. 4, D). Behind the second branchial arteries 

 the follicles decrease in number and size, and completely dis- 

 appear before the third branchial arteries are reached. 



The follicles are circular, oval or irregular in cross section. 

 The diameters of the circular ones vary between 40 and 200)U, 

 the larger ones are more numerous, especially in the anterior 

 region. Branched follicles occur, sometimes as many as five 

 follicles leading into a larger one. 



Here also the follicular epithelium is low, almost flat, and the 

 follicles are completely filled with homogeneous colloidal sub- 

 stance. Sometimes, however, the colloid contains particles, 

 probably destroyed blood corpuscles or epithelial cells. The 

 blood supply is rich, many capillaries lying close to the follicles. 

 There seems to be a comparatively better circulation here than 

 in the younger stages. 



SALMO MYKISS WALBAUM 



Specimen 11 cm. In the black spotted trout the thyreoid 

 gland shows a great antero-posterior extension. The posterior 

 limit is about that shown by Maurer in a 20 cm. trout, species 

 not named, apparently a brook trout. However, the main part 

 of the gland is situated above the aorta, not below it as Maurer 

 claimed. The anterior limit of the gland lies well in front of the 

 aortic bifurcation and the posterior end behind the third branchial 

 arteries (pi. II, fig. 21). The dorso-ventral distribution is also 

 more pronounced than in most of the other species, especially as 

 to the number of follicles below the aorta. The main mass of 

 the organ lies in the aortic bifurcation (fig. 5, ^). The copula 

 reaches far down and divides it into two halves. Along this 

 cartilage the follicles extend dorsally close up to the floor of the 

 pharynx. Laterally also the extension of the follicles goes as far 

 as possible. 



