FATE OF THE ULTIMOBRANCHIAL BODIES 115 



The deeply stained nuclei have been regarded by Simon ('96) 

 as degenerating nuclei. In only two developmental stages (23 

 and 24 mm. embryos, fig. 7) were degenerated nuclei found in 

 sufficient number to suggest a general degeneration of these 

 structures. In some of the later developmental stages degener- 

 ated nuclei were also found but always in small numbers. It 

 appears that the degenerated nuclei are derived from the darkly 

 stained nuclei although I was unable to trace their source through 

 intermediate forms directly to them. Some of the nuclei in con- 

 nective tissue cells, in developing muscle fibers, in epithelial 

 cells lining the esophagus, and also in some stages in the cell 

 cords of the thyroid gland, stain deeply. This gives them a 

 structural appearance quite similar to those found in the ultimo- 

 branchial bodies. The presence of these nuclei in various develop- 

 mental structures suggested the probability that the dark nuclei 

 in the ultimobranchial bodies are in a certain physiological state. 

 This, however, is mere conjecture. If they represented a gen- 

 eral degeneration of the ultimobranchial bodies one would nat- 

 urally expect to find large numbers of degenerated nuclei in later 

 developmental stages, but a contrary condition is the case. 

 They gradually decrease in number while the ultimobranchial 

 bodies continue in their growth. This fact seems to me 

 to be strong evidence in favor of the persistence of these 

 structures. 



A feature quite noticeable in the ultimobranchial bodies in 

 most of the earlier developmental stages and in some of the quite 

 late stages is the small and variable size of some of the ultimo- 

 branchial nuclei. Grosser ('10) and Kingsbury ('14) also ob- 

 served small ultimobranchial nuclei in human embryos. The 

 small nuclei are very variable in number in stages of about the 

 same age. Some of these nuclei also stain deeply in stages in 

 which deeply stained nuclei are present, and in a few instances 

 groups of pale small nuclei were found. However, the large ma- 

 jority of the small ultimobranchial nuclei have a normal structure, 

 in all stages in which they occur. In late stages no small nuclei 

 are present. Although the significance of the darkly stained 

 and the small ultimobranchial nuclei are unknown to me I am 



