114 J. A. BADERTSCHER 



found in their more proximal ends near their attachment to the 

 more central unbroken portion of these bodies. Also, in stages 

 in which the darkly stained nuclei are numerous many can usu- 

 ally be found in the coarse cell cords. The presence of nuclei in 

 mitotic "division in these cords is further evidence that they really 

 grow. 



The cell cords of the ultimobranchial bodies when first formed 

 are generally more loosely arranged than those of the thyroid 

 gland (figs. 9, 10 c, and 13). The time at which they become 

 more compactly arranged and resemble in appearance the thy- 

 roid gland previous to the appearance of colloid in the latter, 

 varies greatly. For example, in embryos of 48 and 53 mm. in 

 length, excepting the small vacuolar portions, they have a struc- 

 ture, similar to the thyroid gland, while in both 125 mm. embryos 

 cell cords in portions of these structures have still a quite loose 

 arrangement. 



The deeply stained nuclei are most numerous in the ultimo- 

 branchial bodies in stages from 20 mm. to about 30 mm. in 

 length. In the first half of this brief developmental period (20 

 to 30 mm.) the ultimobranchial bodies attain their largest size 

 as unbroken or solid structures while in the latter half of this 

 period the process of cell cord formation is very active. The 

 deeply stained nuclei diminish in number in stages beyond 30 

 mm. in length and finally disappear altogether. Their decrease 

 in number is, however, not uniform in successively older stages. 

 For example, in a 35 mm. embryo in which the only structural 

 traces left of the ultimobranchial bodies are small disconnected 

 vacuolar areas, the darkly stained nuclei are comparatively few 

 in number, while in a 37.5 mm. embryo in which these structures 

 are still large and easily traceable, the darkly stained nuclei 

 are quite numerous. In late developmental stages in which the 

 ultimobranchial bodes can be structurally recognized as such the 

 darkly stained nuclei have largely or entirely disappeared. For 

 example in No. 2 of the 125 mm. embryos there are some present 

 although not in large numbers, while in the ultimobranchial 

 bodies in No. 2 of the full term embryos no darkly stained nuclei 

 are present. 



