THE EVIDENCE OF THE THYROID GLAND 



20I 



Ammocoetes 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 



\— -v 



Ps.br 



£&**' 



8 X, 



9 X 6 



Fig. 85. — Facial Segment op Ammoccetes maeked 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 



