THYREOID GLAND IN SQUALUS ACANTHIAS 193 



pharynx. The cells which make up this bud are quite like those 

 of the entoderm from which it is derived. The cell outlines are 

 not apparent. The thyreoid nuclei, which are all very similar, 

 contain one, two or three large masses of chromatin, which are 

 round or oval in shape and smooth in outline, except for one or two 

 small chromatin threads which may extend outward from each 

 mass. These chromatin masses are generally closely applied to 

 the nuclear membrane. The remainder of* the nucleus is filled 

 with a clear nuclear sap through which run a few delicate and 

 faintly staining fibrils. This type of nucleus, which has been 

 described and figured by McGill ('10), Neal ('14), and Scammon 

 ('15), is characteristic of the young Acanthias embryo, being 

 found also in the cells of the mesenchyma, mesothelium, medul- 

 lary canal, in the hepatic anlage, and elsewhere. In the earliest 

 stages, all of the nuclei are placed with their long axes perpen- 

 dicular to the surface of the thyreoid bud. 



This condition is present only in the early anlage while the gland 

 mass is diminutive. As soon as the gland increases in size 

 (embryo of 5 mm.), changes are noticeable both in the structure 

 and arrangement of the nuclei. The chromatin masses above 

 mentioned become more irregular and there are given off from 

 them a number of chromatin strands which eventually form a 

 coarse network. The chromatin masses may also be somewhat 

 broken up, being separated from the nuclear wall so as to present 

 a granular appearance. But the change in form of the nuclei is 

 perhaps even more striking than their altered structure. In 

 place of the elongated ovoidal nuclei of the early anlage, which 

 were in all respects like the nuclei typical of the general entoderm, 

 the gland at this stage presents nuclei which appear in the cross- 

 sections to be nearly spheroidal. These nuclei have diameters 

 which are approximately the same as the short diameter of the 

 nuclei found in the general entoderm. Those which are most 

 ovoidal lie in the peripheral part of the gland and are placed with 

 their long axes perpendicular to the surface. The nuclei of the 

 central part of the gland, on the other hand, are very irregularly 

 disposed. At this same stage (embryo of 5 mm.) a large number 

 of vacuole-like spaces can be seen which are quite generally dis- 



THE JOURNAL OF MORPHOLOGY, VOL. 31, NO. 1 



