624 FRANCIS M. BALDWIN 



2. Thyreoid 



The anlage of the thyreoid is first recognized in Amblystoma, 

 as in a number of the other amphibians in which its develop- 

 ment has been studied, (Triton, Maurer '88; Muthmann, '04; 

 Necturus, Miss Piatt '96; Hypogeophis, Marcus, '08; and Axolotl, 

 Maurer, '88; and Muthmann, '04) in the late embryo and early 

 larval stages. Muthmann ('04) describes its earliest appearance 

 in embryos of Triton and Axolotl of 21 somites, but carries his 

 study only to the hatching stage, 30 somites (probably about 4 

 mm. long). In the following account the description of it begins 

 with embryos 5 mm. long, thus roughly corresponding to the last 

 stage described by Muthmann. 



A. Amblystoma larvae, 5 mm. long. At this stage, the general 

 features of the pharyngeal region are not much different from 

 those earlier described in dealing with the thymus bodies. The 

 place where the mouth is to form is easily recognized, being indi- 

 cated by an extensive oral plate, into which there extends from 

 behind, the now spacious pharyngeal lumen, lined throughout by 

 a comparatively thick layer of yolk-filled entodermal cells. The 

 caudal end of the lumen is continuous with the rest of the primi- 

 tive alimentary tract, which dips ventrally in the region caudad 

 to the heart to form the anlage of the liver. As yet the pharyn- 

 geal pouches are indicated by mere grooves along the sides of the 

 pharyngeal tube, the dorsal wall or roof of the tube being com- 

 posed of a rather uniform layer of cells. The thyreoid body arises 

 from certain cells lying in the median floor of the pharyngeal 

 tube, in the region between the oral plate and the heart. 



In the region of the early anlage of the hyomandibular pouch 

 (not the second branchial, as Maurer '88 described in Triton), 

 and in front of the anterior limits of the pericardial wall, the 

 entoderm in the mid-line of the floor of pharynx thickens and at 

 the same time folds in such a way as to form a very shallow cup- 

 like depression which protrudes ventrally and a little caudal be- 

 tween the mesodermal cells of the third somite of the head below. 

 This is the early anlage of the thyreoid, and, because of its close 

 proximity to the heart region in the earlier developmental stages, 



