1902.] NATURAL SCIENCES OF rHILADELFHIA. 99 



triangular iii cross section, and is pai'tially divided into two chambers 

 by the median lamella (in. I.) which rises from the middle of its 

 ventral wall. Into each half of the median ciliated chamber 

 empty one of the lai'ge and one of the small groups or cylinders of 

 gland cells. The smaller cell group, as has been said, at this stage 

 is still in a rudimentary condition. 



The lateral ciliated chamber {l.c.h.) is crescentic in cross sec- 

 tion, the lesser curve of the crescent being formed by the ciliated 

 cells, the greater curve by the cover cells. Into the lateral chamber 

 empty a large and a small group of gland cells. Dorsal to the 

 gland are the two large blood vessels (art. ) and close above these the 

 floor of the pharynx, with the ventral ciliated groove (v. e.g.), now 

 much diminished in depth. Fig. 5k is of a section near the poste- 

 rior end of the gland, passing through the upcurved portion. The 

 reference line m.e.h. begins in the cavity of this upwardly bent 

 part, which is cut through at the point where it is continuous with 

 the central ciliated chamber of the gland. A section just anterior 

 to this would show this upper space as a separate cavity. Dohrn 

 says this marked bend at the posterior end, as Avell as the less 

 marked one at the anterior end, is caused by the growth of the 

 gland being more rapid than that of the space in which it is en- 

 closed. 



The lateral portions of the gland at the point represented in fig. 

 5^ are very small. The central partition (m.l.) is still present, as 

 seen in fig. 5/, and imperfectly at hi. T. in fig. om. The ventral 

 groove (v. e.g.) and dorsal ridge (c.d.r.) are also very plainly seen 

 in fig. dk. The blood vessels are followed easily and are usually 

 filled with large nucleated corpuscles. In the last two sections 

 (figs. ~)k and 51) the thyroid has diminished in size, and the 

 arrangement of the glaud cells into the four gi'oups can be I'ecog- 

 nizcd no longer. The lateral ciliated chamber is reduced to a 

 nearly circular cavity, and the ventral ciliated groove is flattening 

 out gradually, preparatory to changing to the low ridge mentioned 

 in the description of the system of ciliated gi'ooves. The ciliated 

 dorsal ridge is very prominent and partially embraces the aorta in 

 its folds. The cell structure of the body wall and of most of the 

 pharyngeal wall is omitted in these as in most of the preceding 

 figures. 



In the following and last stage of development (tigs, i'ya-k), the 



