28 
:\I0N1CA TAYLOE 
Against such a tlieorj, however^ it must be remembered 
tliat this fortn of valve occurs in Lung-fishes as well as in 
Amia, and that it is more primitive in nature than caeca, a 
(|uite similar valve developing in other cases (cf. various 
insects) . 
Stage 85, etc. — Little more remains to be said. The 
pyloric valve is fully developed in Stage 34. In Stage 36 the 
mouth is provided with unicellular glands and numerous 
papillae. A rectum can also be distinguished, being of 
slightly wider diameter than the rest of the intestine and its 
commencement being marked by a sphincter muscle. 
The yolk-mass gradually diminishes, being pushed into a 
dorso-lateral (right) position by the encroachment of the 
developing viscera, the subintestinal vein being, however, 
always in close connection with it. To the end^ot' its existence 
it is always surrounded by a yolk-syncytium. 
Thymus aiid thyroid increase in size. The former is a 
dense mass of deeply staining units interspersed with blood- 
corpuscles and is a conspicuous object both in transverse and 
sagittal sections. At the liiTid end of the thyroid a centi-al 
lumen tilled with a structureless colloidal substance is 
surrounded by cells. In the adult condition the liver is 
elongated and unilobed. Its anterior end stretches almost to 
the pericardium, and is on the left side of the alimentary 
canal, but together with the great length there is a slight 
twisting of the long lobe, so that for part of its course this 
.lies ventral to the alimentary canal, while its hind part (which 
reaches almost to the anus) is on the right of the intestine. 
The gall-bladder is large and conspicuous. It lies in the 
liver, its ventral surface being just fiush with that of tlie 
liver. 
The adult pancreas is a comparatively compact and very 
much elongated gland. It reaches anteriorly to the gall- 
bladder, circumscribes the alimentary canal at the pylorus, in 
which place it attains its greatest bulk, gradually dying out in 
the hinder intestinal regions. The anterior part is dorso- 
lateral, the ])Osterior part beyond the pyloric valve ventro- 
