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Development of the thyroid gland in Petromyzon. 



(llth day.) The thyroid begins, in Petromyzon, as 

 a simple pit or groove, pushed out from the ventral wall of 

 that part of the digestive cavity which may be called the pha- 

 rynx, though at this time there are no gill openings connecting 

 this cavity with the exterior. 



This groove, or beginning thyroid, "as first found, 

 in tlv sr P. planeri .mu L ^i Ii' jI" , in an embryo eleven days old, that 

 is to say, eleven days after fertilization. 



The external appearance of this embryo is seen in 

 Fig. 1 a. At this stage there is no external indication of gill- 

 slits, the mouth Is deeply pitted In, though not yet broken 

 through, and the yolk stilil forms a large, round mass at the 

 posterior end of the embryo. As is seen by the figure, the 

 embrjo is distinctly retort-shaped, the yolk-mass forming the 

 body of the retort, while the neck of the retort is represented 

 by the head and neck regions of the embryo. The stomodaeum is 

 seen as a pit in the head region, and, at a slightly older stage, 

 the first external indication of gill-clefts is seen as a series 

 of shallow transverse grooves in the narrowest part of the neck 

 of the flask. 



The thyroid groove, at its anterior end, is broad and 

 deep, so that, with its thick walls, it is half as large, in 

 cross section, as the part of the pharynx from which it is de- 



