86 PROCEEDINGS OF THE ACADEMY OF [Feb., 



gland cells. Embryos were fixed iu a number of ways, but in no 

 single case was a good preparation of the gland cells obtained. 

 Maceration was tried also, but with no success. Some difficulty was 

 experienced also in sectioning the earlier stages, on account of the 

 mass of yolk, but with later stages there was no trouble of this 

 kind. The figures referred to are on Plates IV-VII. 



All the sections were drawn with a camera, though in most cases 

 the details had to be filled in from a series of sections. 



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 Avail of that part of the 

 digestive cavity which may be called the pharynx, though at this 

 time there are no gill openings connecting this cavity with the 

 exterior. 



This groove, or beginning thyroid, was first found, in P. planeri, 

 in an embryo eleven days old — that is to say, eleven days after 

 fertilization. 



The external appearance of this embryo is seen in fig. la. 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 still 

 forms a large, round mass at the posterior end of the embrj^o. As 

 is seen by the figure, the embryo 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 stomodseum 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 derived. lis Jateral walls, 

 throughout its length, are very thick, and are composed of a single 

 row of greatly elongated cells, each cell with a large nucleus near its 

 centre. The ventral Avail is usually much thinner and is composed 

 of shorter, even cubical, cells. 



As the groove is followed posteriorly, it is found to become gradu- 

 ally narrower and also somewhat more shallow until it finally dis- 

 appears. 



