COMMON FROG. 95 



scarcely yet observable, and shortly afterwards the eye, in a 

 rudimentary state, may be just discerned. The next step 

 in the developement is a slight division of the branchise into 

 lobes, and the appearance of that beautiful and interesting 

 phenomenon, the circulation of the blood through these 

 organs. The embryo, which is still confined to a curved posi- 

 tion by the envelopes (fig. 1), is now seen frequently to extend 

 itself by sudden jerks, as if to emancipate itself from its con- 

 finement, which it eventually does by tearing the membrane 

 of the egg. The Tadpoles, we see, are now hatched (fig. 5). 

 According to Rusconi, this takes place within four days after 

 the deposit of the eggs, in a temperature varying only from 

 23 to 27 Centigr. (73'4 to 80'6 Fahr.) ; but certainly in 

 our own climate, in the ordinary temperature of our spring, it 

 does not take place until at least a month. As I before 

 observed, the developement may be greatly hastened by a 

 constant elevation of temperature, and I was often surprised 

 at the degree to which this was the case in the course of my 

 own observations ; but the comparatively low temperature of 

 our spring, and the cold frosty nights which occur in March 

 and April, may well account for such a discrepancy. If the 

 eggs be deposited at the middle or latter end of March, it is 

 generally, with us, the latter end of April before they are 

 hatched. The situation of the mouth is now distinctly 

 visible, though the opening is but small ; and even when it 

 becomes considerably enlarged it has no power of movement, 

 and there are no distinct lips. The branchise now speedily 

 enlarge, and each of the two branches on either side is seen to 

 consist of about four leaves (figs. 6. 9) ; these are sessile upon 

 the body or stem of the branchia ; they are somewhat granu- 

 lar on the surface, and slightly irregular in their form ; there 

 is also frequently a short additional branch at the base of the 

 posterior one, as is shown in the outline, figure 9. The 

 present state of these organs, which have now arrived at their 



