DEVELOPMENT OF THE TADPOLE. 67 



date, whilst Figs. 2 and 3 are vertical and horizontal longitudinal 

 sections two days later (March nth), and Fig. 4, a section of 

 March 12th. Here it will be observed that brain, somites, noto- 

 chord, and claspers have all undergone a continuous and marked 

 increase in development, and in addition we have a mouth-cavity 

 forming, together with the pericardium, and an immature heart 

 within it. The notochord is well seen in Fig. 2 embraced by a 

 number of the somites ; whilst in Figs. 3 and 4 there are distinct 

 indications of both cerebrum and cerebellum, together with some 

 of the organs of special sense, as the auditory and optic sacs. 

 The pro-renal ducts, commencing just under the gill-arches and 

 running down the whole length of the body, merging at last into a 

 thin line of mesoblast, are also well seen in Fig. 3. 



Not less important than either are the distinct advances made 

 in the growth of the gills. In Figs. 3 and 4 we see the gill-arches, 

 with the gills within them, well protruding from the sides of the 

 head. In the living animal it was not possible at this date to 

 observe anything like a circulation or any special aeration of 

 blood-corpuscles occurring in these immature gills. Indeed, from 

 the extremely elementary condition of the heart, it is not probable 

 that blood had yet commenced to form from any of the nucleated 

 cells. 



In order to see whether it were possible to throw any light 

 on the physical changes occurring during development, it was 

 attempted to take the specific gravity of the animals at various 

 stages of their progress. Peculiar difficulties presented themselves 

 during this part of the investigation owing to the enveloping 

 batrachin attaching itself with the greatest pertinacity to the 

 enclosed embryo. It has been previously mentioned that the 

 specific gravity of the envelope itself was as nearly as possible 

 that of water or very slightly less. Whilst this was the case, it 

 was found that the embryo had a specific gravity decidedly greater 

 than that of water, and varying also from day to day according to 

 the progress of the development. The first attempts at obtaining 

 the specific gravity were made by immersing the embryos, freed 

 as far as possible from their envelope, in solutions of salt of 

 various densities. This, of course, stopped all life ; therefore, in 

 the later stages, it was attempted to obtain correct results by 



