-25- 



would show this upper space a-, a separate cavltj'. Dohrn says 

 this marked bend at the posterior end, as well ns 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 repre- 

 sented in Fig, 5'k are very small. The central partition (m.l.) 

 is still present, ps seen in Fig, 5 1, and imperfectly at 

 m.l'. in Fig. 5 m. The ventral groove (v. e.g.) and dorsal 

 ridge (c.d.r.) are also very plainly seen in Fig. 5 k. The blood 

 vessels are follov/ed easily and are usually filled with large, 

 nucleated corpuscles. In the last two sections C^igs. 5 k and 

 5 1), the thyroid has diminished in size, and the arrangement of 

 the gland cells into the four groups can be recognized no longer. 

 The lateral ciliated chamber is reduced to a nearly circular 

 cavity, and the ventral ciliated groove is flattening out grad- 

 ually, preparatory to changing to the low ridge mentioned in the 

 description of the system of ciliated grooves. The ciliated dor- 

 sal 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 v/all is omitted in these as in most of the pre- 

 ceding figures. 



In the following and last stage of development (Figs, 6 

 a-k) , the gland has apparently reached its greatest complexity. 



