-4- 



rlved. Its lateral walls are, throughout its length, ver^ thlok, 

 and are composed of a single row of greatly elongated cells, 

 each cell v.'lth a large nucleus near its centre. The ventral 

 wall is usually much thinner and is composed of shorter, even 

 cubical, cells. 



As the groove is followed posteriorly, it is found to 

 ■fcecome gradually narrower and also somewhat more shallow until 

 it finally disappears. 



Fig. 1 c, though representing a slightly older stage, 

 may be taken to represent a transverse section through the mid- 

 dle of the gland very shortly aftar its first appearance as 

 an evaginatlon from the floor of the pharynx (phar.). The thick 

 lateral walls and much thinner floor of the thyroid are shown 

 in this section. Anterior to the point represented in this 

 figure, the thyroid groove is somewhat wider, while posterior 

 to this point it is slightly narrower and gradually becomes 

 more and more shallow until it finally disappears. 



The thyroid at this stage, then, is a groove, relative- 

 ly larger, perhaps, than at any other time, and opening along 

 its entire length into the pharynx, 



(12th day.) On the twelfth day the thyroid begins to 

 be shut off from the pharynx by the growth of two horizontal 

 partitions, one roofing over the anterior end, the other roof- 

 ong over the posterior end, of the thyroid groove. 



