-o- 



Indloated in this figure. The cells oP the body wall are very 

 similar in size and shape to those lining this part of the 

 pharynx. Fig, 1 c. is a section cut posterior to Fig, lb,, 

 (through the line a'b'. in Fig, 3) and passes through the part 

 of the thyroid which is still open above to the pharynx. In 

 Fig. 1 b, only the cells of the thyroid are dotted, while in 

 Fig. 1 c. the cells of both thyroid and pharynx are dotted. 

 The only change since the eleven-day stage is the beginning 

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

 converted into a closed tube by the backward growth of the ante- 

 rior 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 pharj^nx, causing 

 these tv?o projections to be pushed out, the upper layer of each 

 septum apparently being derived from the pharyngeal epithelium, 

 the lower from 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, 1 b., the dorsal wall of the thyroid is closely pressed 

 against the ventral wall of the pharynx, £0 that it is difficult 

 to distinguish the two walls. 



It will be interesting to compare the^Petromyzon^^hy- 



roid at this stage v/ith the thyroid, at a corresponding stage, 

 in Amblystoma (Figs. 8 a and R b). A section through the 



