FATE OF THE ULTIMOBRANCHIAL BODIES 99 



roid gland. A few blood vessels of a capillary character are 

 found in the larger portion of the ultimobranchial bodies. 



Embryos of 24 to 30 mm. During this developmental period 

 quite marked changes occur in the ultimobranchial bodies, the 

 most pronounced of which is a breaking up of their major por- 

 tion into cell cords which, when first formed, are usually larger 

 than those of the thyroid gland. Two factors are apparent dur- 

 ing the formation of cell cords, namely, a continued growth and 

 division of the epithelial buds, and their invasion by mesenchy- 

 mal and vascuolar connective tissue. The extent to which this 

 process occurs during this developmental period varies. In some 

 embryos these structures are almost entirely broken up into 

 cell cords while in others a centrally located, more or less vacuolar 

 and irregularly outlined core, variable in size, persists for some 

 time longer. This process is illustrated in figure 9 (U), which 

 represents a section through almost the middle portion of the 

 tripartite complex in a 27 mm. embryo. In most stages of this 

 developmental period (24 to 30 mm.), and even in some later 

 stages, the caudal portion of the ultimobranchial bodies is for a 

 time less broken up into cell cords than their more anterior part. 

 Also, these structures never become entirely vacuolar. Some of 

 the coarse cell cord^ are composed of a nonvacuolar syncytium. 

 Nonvacular areas are also present in the more centrally located 

 syncytial mass and in the larger and less unbroken caudal por- 

 tion of these structures. Groups of small nuclei which appear 

 normal in structure are preseilt in both vacuolar and non vacu- 

 olar parts. In places, instead of being arranged in groups, the 

 small nuclei are quite uniformly scattered among the larger 

 nuclei. Mitoses in both the thyroid gland and ultimobranchial 

 bodies can readily be found. 



It is during this developmental period and also in somewhat 

 earlier and later stages that the darkly stained nuclei are most 

 numerous. In only two developmental stages, namely, in a 23 

 mm, embryo (not the one described above), and in a 24 mm. 

 embryo (fig. 7, D.N.), were degenerated (pyknotic and frag- 

 mented) nuclei found in sufficient numbers to .suggest a general 

 degeneration of these structures. The degenerated nuclei in 



