CIRCCTLATTON IN TADPOLE. 737 



aware, is essentially a Fish in the early period of its existence, 

 breathing by Gills alone, and having its Circulating apparatus 

 arranged accordingly : but as its limbs are developed and its tail 

 becomes relatively shortened, its Lungs are gradually evolved in 

 preparation for its terrestrial life, and the course of the blood is 

 considerably changed. In the Tadpole as it comes forth from the 

 egg, the Gills are external, forming a pair of fringes hanging at the 

 sides of the head (Plate xxiv., fig. 1); and at the bases of these, 

 concealed by Opercula or gill-flaps resembling those of Fishes, 

 are seen the rudiments of the internal Gills, which soon begin to be 

 developed in the stead of the preceding. The external gills reach 

 their highest development on the fourth or fifth day after emersion ; 

 and they then wither so rapidly, whilst at the same time being 

 drawn -in by the growth of the animal, that by the end of the 

 first week only a remnant of the right gill can be seen under the 

 edge of the operculum (fig. 2, c), though the left gill (b) is some- 

 what later in its disappearance. Concurrently with this change, 

 the internal gills are undergoing rapid development ; and the 

 beautiful arrangement of their vascular tufts, which originate 

 from the roots of the Arteries of the external gills, as seen 

 at g, fig. 5, is shown in fig. 4. It is requisite that the 

 Tadpole subjected to observation should not be so far advanced 

 as to have lost its early transparence of skin ; and it is further 

 essential to the tracing out the course of the Abdominal vessels, 

 that the creature should have been kept without food for some 

 days, so that the intestine may empty itself. This starving 

 process reduces the quantity of Red corpuscles, and thus renders 

 the Blood paler ; but this, although it makes the smaller branches 

 less obvious, brings the circulation in the larger trunks into more 

 distinct view. "Placing the Tadpole on his back," says Mr. 

 Whitney, " we look, as through a pane of glass, into the chamber 

 of the chest. Before us is the beating heart, a bulbous-looking 

 cavity, formed of the most delicate transparent tissues, through 

 which are seen the globules of the blood, perpetually, but alternately, 

 entering by one orifice and leaving it by another. The Heart 

 (Plate xxiv., fig. 3, a) appears to be slung, as it were, between two 

 arms or branches, extending right and left. From these trunks (6) 

 the main Arteries arise. The Heart is enclosed within an en- 

 velope or Pericardium (c), which is, perhaps, the most delicate, and 

 is, certainly, the most elegant beauty in the creature's organism. 

 Its extreme fineness makes it often elude the eye under the Single 

 Microscope, but under the Binocular its form is distinctly revealed. 

 Then it is seen as a canopy or tent, enclosing the heart, but of such 



"Transact, of Microsc. Soc." X.S., Vol. x. (1862), p. 1, and his subsequent 

 Paper ' On the Changes which accompany the Metamorphosis of the Tad- 

 pole' in the same Transactions, Vol. xv. p. 43. — In the first of these 

 Memoirs, Mr. W. described the Internal Gills as Lungs ; an error which he 

 corrected in the second. 



3 B 



