563 



of the small adductor arcuum branchialium. The genio-hyoidei, at 

 either side of the thyroid outgrowth, pass from this section anteriorly 

 into a dorsal plane, and posteriorly into a ventral plane. The 

 thyroid outgrowth now bifurcates on the antherior wall of the 

 pericardium. 



add th yh nih 



sh - 



pc 



km el 



- km cl 



add hb cl 



Fig. 7. From two horizontal sections through the thyroid outgrowth in an embryo 

 15 mm long. add adductor arcuum branchiulium. gh genio-hyoideus. hbcl hyo- 

 branchial cleft, hmcl base of hyomandibular pocket, mh mylohyoideus, pc pericardium. 

 *A sterno-hyoideus. th thyroid outgrowth. 



In sections through an embryo of 16 mm, the thyroid outgrowth 

 is unchanged, but immediately after this stage, the outgrowth becomes 

 irregularly broken in its median part so that the anterior and 

 posterior parts are separate. Cells from the anterior part of the 

 broken outgrowth wander downwards onto the dorsal surface of the 

 mylohyoideal muscle, and are finally lost in muscular tissue. No 

 accessory thyroid develops in this region. A few scattered cells from 

 the anterior part of the outgrowth are also found above the level of 

 the muscles genio-hyoidei, but the subsequent fate of these isolated 

 cells could not be followed after their distinguishing yolk granules 

 had disappeared. 



The division of the posterior part of the thyroid, begun in the 

 embryo of 15 mm (fig. 7), continues, and in embryos of 17 and 

 18 mm, one finds in place of the solid thyroid outgrowth, two lines 

 of cells extending obliquely outwards and upwards at the head of the 

 adductor muscles , but forming neither a solid mass nor the walls of 

 closed vesicles, These posterior cells of the thyroid outgrowth appear 

 to migrate dorsalwards as mesenchyme, and may be distinguished in 

 an embryo of 19 mm as a group of cells lying on each side of the 

 embryo, between the lateral surface of the sterno-hyoideus and the 

 cartilage of the first epibranchial, just posterior to its union with the 



