THE EVIDENCE OF THE THYROID GLAND 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- 
eetes, 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 transformaticn the 
thyroid of Ammoccetes 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 Ammoceetes 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 (7h.) 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 
Fig. 81 represents the nature of the 
organ at different levels in front of and 
behind the opening into the respiratory Fis. 80.—Ventran Vimw oF 
chamber ; and in Fig. 82 I have sketched _ es eames 
Th., thyroid gland; M., lower 
the appearance of the whole gland, itn yells tescrarlacle 
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 (7h. 0.) into the pharyngeal 
0) 
