OF THE TADPOLE. Il7 



are also connected with the immature inner gills, and as these 

 grow and take more of the blood proceeding from the heart, so 

 the outer ones receive less, and thus atrophy in consequence of 

 the comparatively less amount of nourishment supplied. These 

 inner gills, he says, are functionally active, almost to the very end 

 of tadpolian life, wlien, from a less amount of blood traversing 

 the gill-crests, and a greater quantity being forced into the 

 cephalic and pulmonary arteries, they lose their form, gradually 

 shrink, and give place to true lungs. 



This is, however, at a much more advanced period than that 

 to which we have now attained, and must be left for another 

 paper. 



EXPLANATION OF PLATES XI. AND XII. 



Plate XI. 

 Figs. 1, 2, 3, and 4. — Different phases in the development of the 

 tadpole, showing vertical sections of March 29th, 30th, 31st, 

 and April 1st, to illustrate some of the changes which occur 

 during the passage of the undeveloped yelk into the coiled 

 intestine, and also of the simple mouth without teeth into the 

 fringed mouth containing several rows of teeth. 

 1 a., lb., 1 f. Phases in the development of the undiffer- 

 entiated yelk with the coiled intestine. 



Plate XII. 



Fig. 5. — Tadpole of April 5th, showing coiled intestine, fringed 

 mouth, muscles of the lower lip, and internal gills. 

 ,, C. — Mouth on larger scale, showing hooked bristles and rows of 

 teeth. 



,, 7. — Teeth, x 240 diameters from upper jaw. 



,, 8. — Enlarged crests of the inner gill, showing the blood-vessels 

 passing around each crest, as well as the more elongated 

 crests at the edge of the gill. ms.L, muscles of lip ; oc., eye ; 

 ms.no., muscles of notochord ; /p., lip ; A. , heart ; /., liver; 

 t/A'. , undifferentiated yelk ; p.r.d., pro-renal ducts ; /(., noto- 

 chord ; i.n., internal nares ; m.c, termination of mouth- 

 cavity; 2*., intestine; bl., bladder; f.b., fore-brain; no., 

 notochord ; t., teeth ; m., mouth. 



