96 PROCEEDINGS OF THE ACADEMY OF [Feb., 



opening of the cell group (fig. og and fig. 61, jj.s), which appeared 

 to be a small mass of secretion which had been forced out of the 

 cell group by the contraction of the gland at fixation. There was 

 no secretion foimd in any other part of the gland, which fact 

 could be explained only by supposing that the secretion was carried 

 away as fast as it was formed, by the cilia. ^Vs seen in fig. 6^ (a 

 camera drawing of a section of the next stage, magnified 925 

 diameters), the opening of the cell group is covered over by a 

 membrane which is continuous with the membrane from which the 

 cilia arise. As this membrane, in any given group, may be fol- 

 lowed without a break through a long series of sections and the 

 protuberance of secreted matter seen in each section, it is difticidt 

 to see how the secretion gets on the outer side of the membrane, 

 unless it passes through a series of holes in the membrane that, 

 when filled as they are with secretion, are too minute to be seen, 

 even under a magnification of 1200 diameters. 



In fig. 6^ may be seen, just under the basement membrane, a 

 number of triangular condensations of the cell substance on each 

 side of the opening. These inverted cones were seen in a number 

 of sections that happened to be stained in just the proper way, but 

 no explanation of their presence suggested itself. 



Roughly speaking, each half of the thyroid may be considered 

 to be made up essentially of two large and two small cylinders of 

 gland cells, each cylinder extending the whole length of the gland 

 (fig. oe, (jr. and (//•'. ). The small cylinders (gr'.) are as yet so 

 rudimentary that they can scarcely be recognized as such, but, as 

 has been said, they w^ill acquire later the same structure as is now- 

 possessed by the larger cylinders (gr.). Extending the entire 

 length of each cylinder or cell group is a narrow baud of minute 

 holes, mentioned above (fig. 5/, o., and fig. 61, ?».), through 

 which the secretion of the gland cells escapes into the ciliated cham- 

 bers and thence, through the duct, to the ciliated grooves of the 

 pharynx. 



The remaining structural details of the gland may be understood 

 from a description of the duct, which is somewhat complicated. 

 As has been described above, the two ventral branches of the 

 ciliated grooves run along the floor of the pharynx, getting closer 

 and closer together, imtil they unite and form a single deep groove 

 (figs, oe-h, v.c.g.). In fig. ")/" the ventral ciliated grooves (v. e.g.) 



