FATE OF THE ULTIMOBRANCHIAL BODIES 117 



as claimed by Simon, there would be some quite large thyroid 

 follicles found in the deeper portion of the ultimobranchial 

 bodies among the smaller ultimobranchial follicles which begin to 

 develop comparatively late. However, excepting the cystoid 

 follicles in the ultimobranchial bodies in some of the later stages, 

 this condition is not found. The follicles containing colloid 

 gradually increase in size from the more central portion to the 

 periphery of these structures. It therefore seems that the con- 

 tention of k^imon is incorrect. 



It also appears that in a few stages by far the larger portion 

 of the ultimobranchial bodies undergo a transformation into 

 typical thyroid structures even before colloid is formed in the 

 thyroid gland. For example in embryos of 35 and 53 mm. in 

 length the only structural features of the tripartite complex that 

 can be interpreted as derivatives of the ultimobranchial bodies 

 are small vacuolar areas (fig. 12) in contrast with the loosely 

 arranged cell cords of these structures as found in embryos 37.5 

 and 60 mm. in length. Since in early stages it is impossible to 

 distinguish the minute structure of the nonvacuolar portions of an 

 ultimobranchial body from that of the thyroid gland when both 

 are seen in the same microscopic field under high magnification, 

 I believe that the vacuolar areas in embryos of 35 and 53 mm. 

 in length represent only the more central cores of ultimobran- 

 chial bodies of which their more peripheral portion has under- 

 gone an early transformation into typical thyroid structures. 

 This interpretation is supported by the conditions presented in a 

 48 mm. embryo in which the anterior portion of each ultimo- 

 branchial body is isolated from the thyroid gland. Excepting a 

 small vacuolar area and traces of a lumen found in the isolated 

 portion of the left one, the isolated portion of each of these 

 bodies has a structure similar to the thyroid gland along which 

 it lies. 



I am of the opinion that the so variable developmental be- 

 havior of the ultimobranchial bodies in pig embryos throws 

 light on a disputed point in connection with the development of 

 these structures in human embryos. Grosser ('10) writes of a 

 'dichtere Zellgruppierung' in the thyroid gland of a human embryo 



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



