2(')2 DKVKLOPMENT OF A XOX-AQUATIC TADPOLE. 



Fig. 3. — A small-sized egg (probably male) with embryo. The beginnings 

 ot fore- and hind-limbs are visible. The fore-limbs are as yet quite 

 exposed and uncovered by any fold. In the case of these small eggs 

 the tail projects straight backwards from a very early stage. x 10. 



Fig. 3a. — A small tadpole (probably male). Note the shape and expansion 

 of median swimming membranes of tail and compare with those of the 

 large tadpole, fig. 4a. X 10. 



Fig. 4. — A large-sized egg (probably female) with embryo. The beginnings 

 of the fore-limbs are covered by a forwardly growing epidermal fold 

 (f.) which must be regarded as an opercular fold whose line of origin 

 has been shifted from the normal position on the hyoid arch. In the 

 case of these large eggs the tail grows coiled against the yolk-mass, and 

 straightening does not occur until relatively late (cf. fig. 3). x 10. 



Fig. 4a. — A large tadpole (probably female). Compare with small tadpole, 

 fig. 3a, and note relatively longer tail and reduced swimming 

 membranes. X 10. 



Plate III. 



Figs. 1 — 5 are drawings of models of the gut, which have been prepared 

 from microscopical serial sections. 



Fig. 1. — Side view of gut and yolk-mass of early embryo when tail and 

 hind-limbs are only just appearing : a. Archenteron, d.f.g. Descending 

 portion of fore-gut, li.c. Hepatic diverticulum, p. Pharynx. x 20. 



Fig. 2. — View from above of gut and yolk-mass (cf. fig. 1). h. Hepatic 

 diverticulum. x 20. 



Fig. 2a. — Transverse section through sub-notochordal rod, archenteron [a] 

 and unsegmented yolk-mass [u.ik). x 75. 



Fig. 2b. — Section through descending portion of fore-gut (d.f.g.) which is 

 being differentiated out of yolk. Embryo is a little older than in 

 fig. 2a and the whole yolk has rapidly broken up into large oval 

 separated cells [y.c.) crowded with yolk-globules. This cellular condi- 

 tion of the yolk is very transient ; very soon many of the large nuclei 

 break down and an apparently continuous yolk-mass again results. X 60. 



Fig. 3. — View from above, showing attachment of fore-gut (a.f.g.) and of 

 hind-gut (n.fi.g.) to the yolk-mass, the middle region of the archenteron 

 tube has disappeared. X 20. 



Fig. 4. — Later stage. Differentiation at the attachments of the fore- and 

 hind-guts to the yolk-mass has progressed. At the same time the yolk- 

 mass is endowed with vitality and is being modelled as a whole into a 

 thick, crude spiral yolk-band passing ventrally, and connecting the two 

 ends of the more perfectly formed gut. p.d. marks the position of the 

 attachment of tlie pancreatic duct to the part of the yolk-band which 

 will be converted into the hinder portion of the duodeiuim. /. Lung 

 sac. X 20. 



Fig. 5. — Gut of metamorphosing frog in which the tail has nearly dis- 

 appeared. The whole of the gut has undergone great modification since 

 the tadpole condition seen in fig. 4. The well-formed portions of 

 fore- and hind-gut of fig. 4, which appeared to be definitive in nature 

 have shortened and thickened very greatly, and the yolk-band has been 

 reduced to a simple U-shaped loop extending horizontally. v.d.o. 

 Wolffian duct openings. X 20. 



Fig. 6. — Dorsal view of gut of adult frog given for comparison with fig. 5. 

 It will be noticed that relatively the gut has elongated considerably. 

 The position of the opening of the pancreatic duct (p.d-) in figs. 4, 5 

 and 6 gives some idea of the fundamental re-modellings and trans- 

 formations of the whole of the gut in the later stages of development. 

 b. Bladder. X 4. 



