90 PROCEEDINGS OF THE ACADEMY OF [Feb., 



It is well to get a clear idea of these lateral invaginations in the 

 pimple condition represented in fig. 26, as the more complicated 

 later stages will thus be more easily understood. Were the lateral 

 walls straightened out, thus obliterating the lateral invaginations, 

 the thyroid would be reduced to a more or less cylindrical body, as 

 it was in fig, 16, though with a much larger cavity. The long 

 cells of the lateral walls are divided by the invagination into two 

 groups, which become more and more distinct as development pro- 

 ceeds. In fig. 26 one of these groups, on each side, lies between 

 the invagination and the perpendicular part of the cavity of the 

 gland ; the other group lies between the invagination and the dorsal, 

 horizontal part of the cavity (compare fig. be). The cavity of the 

 thyroid, between this point and the posterior edge of the opening 

 into the pharynx, is simply a deep and narrow groove slightly 

 expanded at the bottom, where a narrow space is left, on either 

 side, between the thick lateral and thin ventral walls (fig. 2c?). 

 Along this groove-like pai-t of the thyroid the cells of the pharynx 

 and gland pass quite insensibly into one another, and are not sharply 

 distinguishable, as might be supposed from the stippling in figs. 2c 

 and 2d. The portion of the thyroid posterior to the opening is a 

 simple tube-like structure, nearly circular in outline and with a 

 vertical slit-like cavity (fig. 2d). The walls here are of nearly 

 the same thickness on all sides, and lie in conta;ct with the epithe- 

 lium of the pharynx above and with that of the body Avail below: 

 in fact, throughout almost the entire length of the gland this is the 

 case. On each side of the thyroid, throughout its entire length, lies 

 a pseudobranchial arteiy {art.) (" Sprit zlocharterie " of Dohrn). 

 There is no indication, as yet, of the ciliated grooves, " Pseudo- 

 branchialrinne." 



The stages of development which follow were obtained from 

 Ithaca, and the larvse, it will be remembered, were possibly of two 

 species, neither of which was identical with the material from 

 Naples, P. planeri. 



The next change that is noticed in the thyroid gland is the devel- 

 opment of the median vertical lamella (fig. 4a, 7n.l.), which is 

 formed by the invagination of the thin dorsal and ventral walls, 

 beginning at the anterior end and gradually extending posteriorly 

 until, at this stage, the gland is separated into two distinct lateral 

 parts, from the anterior end about half-way to the opening to the 



