196 E. H. NORRIS 



cumferentially disposed row around the periphery of the thyreoid 

 bud, as well as in the cells lining the pharynx. The second type, 

 those which are spheroidal or at most slightly ovoidal, are found 

 both in the center of the thyreoid bud and in its suspending stalk. 

 Structurally, these two types of nuclei are very similar. They 

 possess a distinct nuclear membrane and regularly contain two 

 large, smooth, chromatin masses which do not lie in apposition 

 with the nuclear wall. These chromatin blocks are suspended in 

 a clear nuclear sap through which run a few faintly staining linin 

 fibrils. Certain vacuole-like spaces, similar to those described 

 in the gland of a 5-mm. embryo, are also present in this stage and 

 may be noted in the succeeding members of the series up to 

 approximately 20 mm. 



As shown in figure 4, the gland at this stage (embryo of 5.8 

 mm.) contains a large amount of pigment which is scattered 

 throughout the gland mass, but is accumulated in largest quan- 

 tities in the neck of the bud. This pigment is made up of highly 

 refractive, yellowish-brown granules which are closely packed 

 together in masses or blocks of variable size. Whether this 

 pigment is intra- or intercellular cannot be determined with 

 definiteness. Certain of the blocks seem to be superimposed 

 upon underlying nuclei, while others appear to be lodged in the 

 cellular interspaces. A similar pigment is found in the ectoderm 

 of this same region (fig. 4). This pigmentation of the gland 

 occurs in all the specimens from 5.8 mm. up to about 19 mm., at 

 which time the gland severs its connection with the pharynx. 

 Thereafter the pigment decreases rapidly, a little being present 

 even up to 24 mm. 



Histologic evidence of the rapid growth of the gland during 

 these early stages is seen in the large numbers of mitotic figures 

 found in nearly every section. In a single longitudinal section 

 through the gland of a 21 mm. embryo, ten cells in karyokinesis 

 were counted. 



At 19 mm. there appear within the gland a number of com- 

 pletely closed cavities. These cavities are at first only tiny 

 clefts, appearing very much as though the cells around them had 

 only pulled apart a little in their formation. These small clefts 



