OF THE TADPOLE. 31 



interesting feature in Fig. i, Plate IV., is the section of the heart 

 and its attachment both to the Hver and the immature lung by 

 means of the vessels shown, the section fortunately passing through 

 both, and showing not only these, but also the ventricle and 

 auricle of the heart. Tracing back the origin of this organ, we 

 find, by reference to the Plates already given in the April, July, 

 and October numbers, that it has been gradually developing to its 

 present fairly perfect condition. In the April number, PI. VIII., 

 Fig. I, immediately above the claspers, <:/., but not particularised 

 by any letter, is a sinuous tube. This, although drawn somewhat 

 too wide, gives a fair idea of the condition of the heart on 

 March 9th. 



Balfour, in his Embryology, Vol. II., states that the " hearts of 

 Amphibia^ Elasmobranchii^ Cydosfomata, and Ganoids, are formed 

 as tubular cavities in the splanchnic mesoblast on the ventral side 

 of the throat immediately behind the region of the visceral clefts. 

 The walls of this cavity are formed of two layers, an outer thicker 

 layer, which has at first only the form of a half-tube : being incom- 

 plete on the dorsal side ; and an inner lamina formed of delicate 

 flattened cells." The true cavity of the heart is formed within 

 this inner lamina, the outer layer giving rise to the muscular wall 

 and pericardium. 



A section of the cavity and pericardium is well seen in 

 Fig. 2, Plate VIIL, of the April number; and in Figs, i, 2, 3, 

 and 4, Plate XIII., of the July number, are indications of the 

 form of the heart in some of its various stages ; Figs, i and 2, 

 although very small, especially indicating that at this stage its 

 form is by no means that to which we usually attach the meaning 

 heart-shaped, but rather has it the form of a short spiral, or almost 

 that of the letter S, thus showing an analogy between the hearts 

 of the amphibia and the pisces. Balfour lays some stress on the 

 resemblance to the Dipnoi. In Fig. 3 of the same Plate, the true 

 cordate form begins to be apparent, but as these sections were 

 all cut from the same animal, this is only the form of one particu- 

 lar section, but gradually the simple tube becomes by a constriction 

 separated into a dorsal and ventral portion, the former constituting 

 the auricle, and the latter the ventricle. The whole of the upper 

 portions of both these sections was filled with the brain, indications 



