88 PROCEEDINGS OF THE ACADEMY OF [Feb., 



lar in i^ize and shape to those lining this part of the pharjTix. 

 Fig. Ic is a section cut posterior to fig. 16 (through the line a'h' in 

 fig. 3) and passes through the part of the thyroid which is still 

 open to the pharynx. In fig. \b only the cells of the thyroid are 

 dotted, while in fig. Ic the cells of both thyroid and pharynx are 

 dotted. The only change since the eleven-day stage is the begin- 

 ning of the horizontal septa, the anterior end of the thyroid being 

 converted into a closed tube by the backward growth of the 

 anterior septum. 



The septa seem to be formed by a rapid growth at the points 

 where the most anterior and the most posterior epithelial cells of 

 the thyroid merge into those of the pharynx, causing these two pro- 

 jections to be pushed out, the upper layer of each septum apparently 

 being derived from the pharyngeal epithelium, the lower fi'om 

 what may be called the epithelium of the thyroid (fig. 3). The 

 thyroid, though closed in anteriorly, is still entirely open above for 

 a greater part of its length. As is seen in fig. 16, the dorsal w^all 

 of the thyroid is closely pressed against the ventral wall of the 

 pharynx, so that it is difficult to distinguish the two walls. 



It will be interesting to compare the thyroid of Petromyzon at this 

 stage Avith the thyroid at a corresponding stage in Amhlystoma 

 (figs. 8a and 86). A section through the anterior end of the thy- 

 roid of Petromyzon (fig. 16) shows it to be a closed cavity, while 

 posteriorly it is still an open groove (fig. Ic). In Amhlystoma, on 

 the other hand, the open groove is anterior to the closed part of the 

 gland, the closing in taking place, apparently, in a i)Ostero-anterior 

 direction. 



(loth and 14th days.) — At about this time the lateral walls of 

 the thyroid, especially toward the anterior end, become greatly 

 thickened by the increase in length of the columnar cells of which 

 they are composed. At the same time an invagination of these 

 thickened walls occurs as a longitudinal groove, pushing inward and 

 upward from the outside of the gland (fig. 26, i.v.). This invag- 

 ination becomes still more marked in later stages, and extends 

 further in a posterior direction. 



At this stage the extreme anterior end of the thyroid is forked, 

 so that a section through this region would show two distinct cavi- 

 ties with thickened lateral and ventral walls (fig. 2a). This is 

 plainly seen in sections and in a wax reconstruction of the thyi'oid 



