THE EVIDENCE OF THE THYROID GLAXD 



193 



Throughout the vertebrate kingdom it is possible to compare the 

 thyroid gland of one group of animals with that of another without 

 coming across any very marked difference of structure right down 

 to and including Petromyzon. When, however, we examine Ammo- 

 coetes, we find that the thyroid has 

 suddenly become an organ of much 

 more complicated structure, covering a 

 much larger space, and bearing no re- 

 semblance to the thyroid glands of the 

 higher forms. At transformation the 

 thyroid of Animoccetes is largely de- 

 stroyed, and what remains of the gland 

 in Petromyzon becomes limited to a few 

 follicles resembling those of other fishes. 

 The structure and position of this gland 

 in Ammoccetes is so well known that it 

 is unnecessary to describe it in detail. 

 For the purpose, however, of making 

 my points clear, I give in Fig. 80 the 

 position and appearance of the thyroid 

 gland (Th.) when the skin and under- 

 lying laminated layer has been re- 

 moved by the action of hypochlorite of 

 soda. On the one side the ventral 

 somatic muscles have been removed to 

 show the branchial cartilaginous basket- 

 work. 



The series of transverse sections in 

 Pig. 81 represents the nature of the 

 organ at different levels in front of and 

 behind the opening into the respiratory 

 chamber ; and in Fig. 82 I have sketched 

 the appearance of the whole gland, 

 viewed so as to show its opening 

 into the respiratory chamber and its posterior curled-up termi- 

 nation. 



The series of transverse sections (1-6, Fig. 81) show that we are 

 dealing here with a central glandular chamber, C (Fig. 81 (6) and 

 Fig. 82), which opens by the thyroid duct (Th. 0.) into the pharyngeal 







Fig. 80. — Ventral View op 

 Head Region of Ammoccetes. 



Th., thyroid gland; M., lower 

 lip, with its muscles. 



