636 FRANCIS M. BALDWIN 



hyoideus and adductors) it has a more lateral and ventral posi- 

 tion close to the ventral wall of the body. It is now between 800 

 and 900 micra long, thig reduction continuing in the two suc- 

 ceeding stages, to between 712 to 775 micra and 550 to 660 micra 

 respectively. To compensate for this shortening, the follicles are 

 crowded together, those at either extremity having apparently 

 been pushed into the central portion by the growth of the sur- 

 rounding parts, the result being that the central portion of the 

 gland is now greatly enlarged and elliptical in section, measuring 

 between 475 to 625 micra in major, and between 225 to 390 micra, 

 in the minor diameter of the ellipse. 



The following description is based upon a wax reconstruction of 

 the gland in this stage, (fig. 43). The follicles are few (fourteen or 

 fifteen in all), but have increased in size; some being now about 

 100 micra long, and between 50 and 75 micra in diameter. The 

 anterior extremity of the gland consists of a single large follicle, 

 which lies dorsal to the wall of the inferior jugular vein, and, like 

 all the others, is full of colloid; Just behind and a little lateral to 

 this, is a second follicle of about the same size, similarly placed 

 upon the wall of the jugular j Following these, numerous other 

 follicles are irregularly arranged about the vein so that in th© 

 caudal region of the gland, the vein is completely surrounded by 

 them. The spaces between the follicles are occupied by small 

 blood vessels and the lymph sacs. 



Later, when the distal ends of the second, third and fourth 

 branchial arches are greatly reduced in length, and the corre- 

 sponding pharyngeal clefts are greatly modified (the third and 

 fourth closed, and represented by short lateral processes of ento- 

 derm ; the second still open but reduced) ; these changes, although 

 not affecting directly the thyreoid, modify the hypoglossal and the 

 hypobranchial musculature and blood vessels, which, in turn, have 

 a direct effect upon this structure. The gland on either side is 

 short and ovate, measuring in the long axis of the body 712 micra, 

 on the right, and 775 micra, on the left, and in section in its cen- 

 tral region varying from 390 to 510 micra in diameter. It lies, 

 as before in the reduced space between the now much enlarged 

 sterno- and mylo-hyoideus and adductor muscles, and is close 



