ALTMEXTARY CANAL IN LEPIDOSIREN AND PROTOPTERUS, 501 
About stage XXXI (Lepidosiren) the narrow isthmus 
which still unites the thyroid to the buccal floor in front 
of the tongue becomes severed, and the organ lies free as a 
rounded mass of coarsely yolked cells. 
Between stages XXXIV and XXXV the thyroid rudiment 
becomes broken up into strands by intruding mesoderm with 
blood-vessels, and a little later the strands are broken up 
into typical rounded follicles with colloidal secretion in their 
interior. 
Tongue. — Reference to the text-figures (7, a — d) illus- 
trating the development of the thyroid is sufficient to show 
that the tongue of the Dipneumona is a primary tongue, like 
that of Urodele Amphibians, except that in this case no gland 
field develops in connection with the tongue, at least up to 
stage XXXVIII. The downgrowth from the solid buccal 
rudiment, from the posterior side of which the thyroid is 
developed, becomes split about stage XXXI, and it is this 
splitting which causes the portion of buccal floor behind it 
to be bounded in front and laterally by a deep cleft so as to 
form a distinct tongue. 
IV. Lung. 
The first rudiment of the lung is seen in text-fig. 8, A, 
which is a ventral view of the pharyngeal region of a Pro- 
topterus of stage XXXII. 
The lung rudiment is seen to form a rounded bulging from 
the pharynx in the mid-ventral line just at the level of cleft 
VI. Pharynx and lung rudiment are alike solid at this 
stage. 
The endodermal lung rudiment grows at first ventralwards, 
and slightly towards the right side (text-figs. 8 and 12, a, 
pages 502,514). Meanwhile the modelling of the oesophagus 
is proceeding ; it becomes more elongated, more slender, and 
becomes displaced more and more towards the left side of 
the body. The lung rudiment soon begins to bend somewhat 
dorsally, and then continues to grow directly backwards. It 
