FATE OF THE ULTIMOBRANCHIAL BODIES 101 



median and lateral elements of the tripartite complex is not 

 always evident. A description of a few stages will suffice to 

 bring out the general character of the ultimobranchial bodies dur- 

 ing this developmental period. Since the thyroid gland in previ- 

 ous stages is free from vacuoles, it is, I believe, safe to assume 

 that the vacuolar areas found in the succeeding stages represent 

 portions of the ultimobranchial bodies. 



Embryo of 33 mm. The ultimobranchial bodies are limited 

 to the posterior half of the tripartite complex. They are lo- 

 cated on each side of the median plane, deeply buried beneath 

 the dorsal surface of the thyroid gland and are represented by 

 disconnected vacuolar areas the majority of which are not sharply 

 circumscribed but gradually give place to the compactly arranged 

 cell cords of the thyroid gland with which they are fused. The 

 thyroid terminates posteriorly in two short blunt processes. In 

 these processes small vacuolar areas are promiscuously scat- 

 tered among the cell cords. A few" small vacuolar areas which 

 are round in cross section and sharply demarcated by connective 

 tissue from the surrounding cell cords were also found. Only a 

 few darkly stained nuclei are present. No degenerated nuclei 

 were found. 



Embryo of 35 mm. The only traces of the ultimobranchial 

 bodies are small disconnected vacuolar areas on each side of the 

 median plane of the thyroid gland. The gland terminates pos- 

 teriorly in two short blunt processes of nearly equal length, both 

 of which are partly vacuolar. Only a few deeply stained nuclei 

 are present. 



Embryo of 37.5 mm. The anterior portion of the tripartite 

 complex is very large and strongly crescent in outline. Caudal- 

 ward it gradually loses its crescent outline and ends in a single 

 blunt cone-shaped process. The ultimobranchial bodies lie in 

 the posterior four-fifths of the thyroid gland. Their anterior 

 ends lie imbedded beneath the dorsal surface of the thjrroid 

 lateral to its median plane. Caudal ward they rapidly increase in 

 size and shift in position so that in places they extend to the free 

 surface on the lateral margin of the thyroid gland. Their pos- 

 terior ends are fused and compose by far the largest part of the 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 23, NO. 1 



