212 THE DEVELOPMENT OF THE TADPOLE. 



formed, grows rapidly, pushing the ectoderm in front of it, till in 

 about a month (April 30th) it has formed quite a pouch on either 

 side of the protruded intestine, immediately beneath the tail. 

 These pouches, filled with closely packed dividing cells, have 

 exactly the appearance of two small egg-bags, reminding one very 

 forcibly of the egg-packets attached to the tails of some crustaceans. 

 At this early period there is not much difference to be observed in 

 the character or form of the cells of which these pouches are 

 composed, with the exception that possibly the outer layers of 

 cells are smaller than the inner. 



The sac is composed of two layers of cells, the innermost con- 

 sisting of a cubical layer. At the extreme end of this pouch 

 there soon commences to be observed a difference in the character 

 of the cell layers, larger cells appearing there than elsewhere, and 

 thus, appears to me, commences to be produced that wonderful 

 complexity of muscle and nerve, tendon, bone, and cartilage, 

 which we term the leg. These pouches are, thus far, perfectly 

 oval below, shewing no signs of any inequality; but from the large 

 cells, just mentioned, pushing outwards unequally, the lower 

 portion becomes somewhat crenate, or broken up into small 

 irregular knobs, and at the same time points of ossification can be 

 observed in three or four distinct centres, notably in the thigh and 

 carpus, as shewn in Figs. 4, 5, 6, PI. XXII. Around these centres 

 of bone formation the cells arrange themselves, lengthening out to 

 form muscles. The general form of the leg can be well made 

 out by the middle of May, very short and immature certainly, but 

 yet a leg to all intents and purposes, if no use yet either for 

 swimming or jumping, in both which accomplishments the 

 mature animal is so proficient, but yet a leg so fairly formed, 

 that my little son, who has just looked into the microscope, 

 exclaims, " Oh ! there's a leg. I can see it wath my eye." 



One point, however, about this leg is that I can only find four 

 toes instead of the five, which the animal ultimately possesses; three 

 fairly well formed, and the fourth just visible, but whether it is that 

 my sections will not permit me to see the fifth protuberance, or 

 that in reality it is not there, and thus cannot be seen, I am not 

 sure, but incline to the latter explanation. I speak of toes, but 

 in reality they are only distinct protuberances on the growing leg. 



