THYHEOID GLAND OF THE TELEOSTS 759 



seen only a few follicles far to the sides of the aorta and skeletal 

 parts. Behind this the two hypobranchialia have retracted a 

 little from the copula and very small follicles appear in the crev- 

 ices between them. As the copula retracts from the aortic stem, 

 more follicles appear on the dorsal side of the aorta. At the 

 second branchial arteries the follicles cease (pi. I, fig. 4). The 

 follicular epithelial cells are low cuboidal with the longer axis 

 parallel to the base. The colloid appears homogeneous. 



SIPHOSTOMA FUSCUM STORER 



Specimen 30 cm. long. In the pipe-fish the thyreoid gland con- 

 sists of entirely isolated follicles, lying above and to the sides of 

 the aorta (fig. 9). The external form of the fish influences, of 

 course, the form of its inner organs. The thyreoid gland has not 

 found room for dorsal, ventral or lateral expansion and there- 

 fore extends far backwards as a rather narrow streak. The anter- 

 ior end lies at the aortic bifurcation and the posterior end close 

 to the bulbus arteriosus (pi. I, fig. 5). Thus we have a condition 

 in which the organ reaches further towards the tail than usual 

 and where the thyreoid region tapers towards the head end, while 

 as a rule the reverse is true. The number of follicles is not very 

 large, five or six to the section behind the aortic bifurcation. The 

 number decreases towards the second branchial arteries and still 

 more so towards the third, where a transverse muscle occupies 

 the space between the bones and the aorta. At this place there 

 are only one or two follicles in a section, yet there is a continuous 

 chain of them. Near the third branchial arteries the aorta goes 

 down ventrally, the transverse muscle has decreased, and thus 

 the thyreoid finds more space for development. There are six 

 or eight follicles in a section and they lie between the third gill 

 branches which run dorso-laterally. Behind this place the dis- 

 persion of follicles increases (pi. IV, fig. 7), although the aorta 

 lies far ventrally, a fact showing that the thyreoid follicles do not 

 necessarily use the aortic stem as a migration path. On each 

 side of the median line a muscle runs in an antero-posterior direction 

 upon and under which the thyreoid follicles lie. The ventral 



