114 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



a rule, they enlarge as development proceeds, varying from 19 to 

 36 micra in diameter in the 19mm. larvae, to 100 miera in length 

 and between 50 and 75 micra in the late larval and early trans- 

 forming stages. 



A sort of membrana propria appears late in the larval life; 

 it not only surrounds the follicles and the intervening lymph 

 spaces, but also envelops the inferior jugular vein in the region 

 of the gland. 



A so-called rete mirabile of the inferior jugular vein fails to 

 develop during the whole of the larval period, and so far as the 

 \\aiter is able to learn from careful study of the developmental 

 stages, the external carotid artery has no direct connection with 

 the gland. 



Colloid appears rather late in the larval stages, the first evi- 

 dence of it being seen in larvae 39mm. long. 



With the transformation of the ventral gill-region, the thyre- 

 oid gland is pushed a little laterally and caudally, so that it lies 

 in the space just in front of the anterio-lateral wall of the peri- 

 cardium, and, in the adult, it is flanked medially by the genio- 

 hyoideus, laterally by the hyoideus intemus of the first branchial 

 arch, and dorsally by the stemohyoideus muscles. 



From the condition of the follicles in the late transforming 

 and early adult gland, it is evident that new follicles of smaller 

 size are being budded from the larger ones. 



The blood supply of the adult gland is from small venous 

 twigs from the sterno and genio-hyoideus muscles, which upon 

 entering the gland break up into numerous smaller vessels (rete 

 mirabile) and which join ihe jugular vein some distance caudad. 

 Its connection with the external carotid artery is very doubtful. 



The earliest stage in which the anlage of the postbranchial 

 body is recognized in my material is in larv'a^ 8mra. long. It 

 develops as a rule, on the left side (in a single individual 19mm. 

 long on the right side as well). In its earl.v condition, it is a 

 thickening of a certain portion of the pharyngeal floor, lying 

 between the anlage of the fourth branchial (fifth visceral) 

 pouch and the anlage of the glottis, and protruding slightly 

 ventrally towai-d the dorsal wall of the pericardium. It is thus 

 ' postbranehiar as Maurer claimed for a similar structure in 

 Triton, and appears in the relative position of a sixth pouch. 



In 9.5mm. larva», the anlage makes a solid cylindrical stalk 

 of cells (about 40 miera long, and ])etween 25 and 30 micra 



