VEKTEBKATED ANIMALS. 289 



transparence of these parts adapts them, well for this purpose in one 

 respect, yet the comparative scantiness of their blood-vessels prevents them 

 from being as suitable as the Frog's web in another not less important 

 particular. One of the most beautiful of all displays of the circulation, 

 however, is that which may be seen upon the yolk-bag of young Fish 

 (such as the Salmon or Trout) soon after they have been hatched; and as 

 it is their habit to remain almost entirely motionless at this stage of their 

 existence, the observation can be made with the greatest facility by means 

 of the Zoophyte-trough, provided that the subject of it can be obtained. 

 Now that the artificial breeding of these Fish is largely practised for the 

 sake of stocking rivers and fish-ponds, there can seldom be much diffi- 

 culty in procuring specimens at the proper period. The store of yolk 

 which the yolk-bag supplies for the nutrition of the embryo, not being 

 exhausted in the Fish (as it is in the bird) previously to the hatching of 

 the egg, this bag hangs-down from the belly of the little creature on its 

 emersion; and continues to do so until its contents have been absorbed 

 into the body, which does not take place for some little time afterwards. 

 And the blood is distributed over it in copious streams, partly that it 

 may draw into itself fresh nutritive material, and partly that it may be 

 subjected to the aerating influence of the surrounding water. 



686. The Tadpole serves, moreover, for the display, under proper 

 management, not only of the capillary but of the general Circulation; 

 and if this be studied under the Binocular Microscope, the observer 

 not only enjoys the gratification of witnessing a most wonderful specta- 

 cle, bub may also obtain a more accurate notion of the relations of the 

 different parts of the circulating system than is otherwise possible. 1 The 

 Tadpole, as every naturalist is 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 prepara- 

 tion 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 inter- 

 nal 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 being at the 

 same time 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 somewhat 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 exter- 

 nal 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 trac- 



1 See Mr. Whitney's account of * The Circulation in the Tapdole,' in " Transact, 

 of Microsc. Soc.," N. S., Vol. x. (1862), p. 1, and his subsequent paper ' On the 

 Changes which accompany the Metamorphosis of the Tadpole ' in the same Trans- 

 actions, Vol. xv., p. 43. In the first of these Memoirs Mr. W. described the inter- 

 nal gills as lungs, an error which he corrected in the second 

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