1902. j NATURAL SCIENCES OF PHILADELPHIA. 105 



vious sections, and Ls divided by tlie median lamella (m.l. ). In its 

 dorsal position (m.c.g'.) it is a large chamber undivided by a 

 median lamella. In the middle of the coil, especially in the posi- 

 tion lettered m.c.ff", the chamber is much diminished, apparently 

 by the pressure of the surrounding whorls of the coil. By remem- 

 bering that the groups m.c.g'. and m.c.g'". are inverted, as a 

 comparison with the line/, in fig. 6 A* will show, it will be seen that 

 the gland is in reality no more complicated than it was further 

 forward. Nothing further need be said of this section. 



Fig. 6^ represents a section cut through the point where the 

 posteriorly directed whorl (m.c.g".) turns dorsal ward, to pass ante- 

 riorly as the innermost whorl (m.c.g'". ) of the coil. 



Fig. 6h shows the most posterior of the four sections passing 

 through the coil. It passes through the point where the median 

 cell-groups (m.c.g.) make their first bend dorsal ward to where they 

 turn again to form the anteriorly directed whorl (m.c.g'.). 



As has been previously said, the lateral cell-groups (Leg.) take 

 no part in the posterior coil but extend for a considerable distance 

 posterior to the coil, somewhat diminished in size, and gradually 

 approach each other until they are in close contact. 



Fig. 6i represents a section taken at a point about midway 

 between the extreme j^osterior end of the gland and the posterior 

 surface of the coil. It shows the two pairs of lateral cell groups 

 (Leg.), each with its characteristically curved lateral ciliated 

 chamber (Lc.h.). Just posterior to the coil the two large pseudo- 

 branchial arteries, which wei'e seen on each side of the gland in all 

 the preceding sections (figs. 6a-h, art. ), unite to form the one 

 large artery seen in fig. 6i, art., lying between the gland below and 

 the ciliated groove above. This artery is very large, being almost 

 as great in cross section as the end of the thyroid below it. The 

 wall of the pharynx is separated from the gland by a comparatively 

 small space, so that it is evidently pushed upward by the large 

 coiled end of the gland and sinks down again just behind this eleva- 

 tion. The groups of gland cells have diminished somewhat, in cross 

 section, but they are not greatly diminished even at their extreme 

 posterior end. 



If, as Dohrn says, the coiling of the posterior end of the tliyroid 

 gland is caused by its longitudinal growth being more rapid than 

 the growth of the space in Avhicli it lies, it is difficult to see why 



