ALIMENTARY CANAL IN LEPIDOSIREN AND PROTOPTEEUS. 517 
shown by W. N. Parker, it remains enclosed within the 
splanchnic mesoderm ensheathing the gut, neither bulging 
into coelotn nor spreading in the substance of the mesentery. 
As a consequence the pancreas remained undetected by the 
earlier investigators. 
VI. Summary. 
1. The fore-gut first becomes folded off from the main mass 
of yolk-cells. 
2. The pyloric valve arises by the hind end of the fore-gut 
being pushed back into the cavity of the mid-gut. 
3. The main mass of yolk-cells becomes gradually 
“ modelled” into a spirally-coiled intestinal rudiment. 
4. The main part of the buccal lining- is developed in situ 
from large yolk-cells. 
5. The part of the ventral side of the head, on which are 
the olfactory rudiments, becomes enclosed in the buccal 
cavity by the development of the upper lips and by the for- 
ward growth of the lower jaw. 
6. The olfactory opening becomes divided into anterior and 
posterior nares by the apposition and fusion of the inter- 
mediate portion of its lips. 
7. The thyroid arises as a solid downgrowth from the 
buccopharyngeal floor, which gradually becomes cut off from 
behind forwards. 
8. The tongue is a primary tongue like that of Urodeles, 
but without gland-field. 
9. The lung arises from a solid mid-ventral rudiment. 
10. When the lung becomes bilobed, the (actual) right 
lobe is for a time small in size as compared with its fellow. 
11. Complicated torsional processes take place during the 
development of the lung. 
12. Through the dorsal mesentery becoming partially 
merged in the splanchnocoele roof, the lungs come to lie 
outside the splanchnocoele. 
