668 FRANCIS M. BALDWIN 



tween the thickened oral plate, and the anterior limits of the 

 pericardium. 



8. As development proceeds, this cup-like anlage proliferates 

 cells from its ventral surface forming a 'solid cord of cells which 

 extends caudally toward the anterior wall of the pericardium, but 

 it does not unite with the latter in any way. A little later the 

 distal extremity of this cord divides into right and left halves, 

 the division being completed in 10 mm. larvae, and in addition, 

 the connection with the pharynx is lost; the two halves lie lateral 

 to the geniohyoideus muscle of either side. 



9. I find no evidence that the cells which formed either the 

 isthmus of the two halves of the thyreoid, or those which formed 

 the connecting stalk persist to form accessory thyreoids. 



10. After division into right and left halves, each half loses its 

 solid and compact condition. The epithelial cells scatter loosely 

 into the connective tissue and arrange themselves irregularly 

 about the dorsal wall of the inferior jugular vein of either side 

 and from these loosely scattered cells the follicles of the gland 

 subsequently arise by mitotic division. 



11. Follicles first appear in larvae 15 mm. long; they .are either 

 globular or elliptical, with a well defined outer layer of cuboidal 

 cells enclosing a conspicuous cavity, probably containing a fluid 

 of some sort but no colloid. They vary greatly in size, but, as a 

 rule, they enlarge as development proceeds. 



12. A membrana propria appears late in the larval life; it sur- 

 rounds not only the follicles and the intervening lymph spaces, 

 but also envelops the inferior jugular vein in the region of the 

 thyreoid gland. 



13. A so-called rete mirabile of the inferior jugular vein fails to 

 develop during the whole of the larval period, and the external 

 carotid artery has no direct connection with the gland in any of 

 the larval stages. 



14. Colloid appears rather late in the larval stages, the first 

 evidence of it being seen in larvae 39 or 40 mm. long. 



15. With the transformation of the ventral gill-region, the 

 thyreoid gland is pushed a little laterally and caudally, so that it 

 lies in the space just in front of the anterio-lateral wall of the 



