THE EVIDENCE OF THE THYROID GLAND 201 
Ammoccetes is eaten up, and nothing is left except a small, totally 
different glandular mass; and now the cartilaginous basket-work 
meets together in the middle line in this region as well as in the 
more posterior region, In other words, the striking characteristic 
Ps.br 
Fig. 85.—Factan SEGMENT OF AMMOCQHTES MARKED OUT By SHADING. 
VII. 1, thyroid part of segment; VII. 2, hyoid or branchial part; 3-9, succeeding 
branchial segments belonging to IXth and Xth nerves; V, the velar folds; 
Ps. br., Dohrn’s pseudo-branchial groove; Th. o., thyroid opening; C, curled 
portion of thyroid. 
of transformation here is the destruction of this interpolated seg- 
ment, and the resulting necessary drawing together ventrally of 
the branchial segments on each side. 
Moreover, another most instructive piece of evidence pointing in 
the same direction is afforded by the behaviour of the ventral epithelial 
