THE DEVELOPMENT OF THE TADPOLE. 213 



The web, however, soon becomes plainly visible, and is seen to 

 consist of portions of the ectoderm, which remain thinned out and 

 unabsorbed. This is apparently the true explanation of its trans- 

 parency ; not being derived from the true skin it never becomes 

 so crowded with colour cells as is the case with that structure. 



Although by the end of May it is quite easy to make out the 

 shape of the leg-bone, and even to see the form of the head of 

 the femur ; it is not possible at this date to make out the bones of 

 the pubis, which soon afterwards become so prominent a feature 

 in horizontal sections (Fig. 7, PI. XXII. ). There are certainly two 

 points of ossification to be seen just outside the inner portion of 

 the head of the femur, and as many of the muscle-cells from the 

 leg appear to coalesce and terminate at these points, they must be 

 considered as the representatives of the pelvic bones at this stage. 

 The leg, as a whole, is, however, at this period not articulated to 

 any portion of the vertebral column, but is attached above to the 

 true abdominal muscles of the skin, whilst the skin on the outside 

 of the thigh is a continuation of the ectoderm, and that on the 

 inside is connected with that portion of the intestine which pro- 

 trudes beyond the general contour of the abdomen. At this 

 date also the five toes are distinctly visible, the three inner ones, 

 which were the first to form, being much larger than the two 

 outer. At this period the leg-bone, os cruris, as it is afterwards 

 called, can be distinctly seen to be composed of two bones, the 

 tibia and fibula, which are separate throughout the greater part of 

 their length, merely touching at the articulations. 



The warm weather of June appears to exercise a wonderful 

 influence on the rapidity of development ; the tail begins to be 

 absorbed, and as this goes on the true vertebral column can be 

 seen in course of formation, and, indeed, the animal now appears 

 to actually live upon its tail. It certainly eats but little, if at all, 

 during this period of tail absorption, and in order to determine 

 the gain or loss of weight, certain animals were carefully weighed 

 on various days. Thus, an animal with tail half absorbed 

 weighed 3*3 grains ; the same with tail nearly absorbed weighed 

 only 2-6 grains, and although the following determinations were 

 made on an animal about a month later they all give the same 

 history. Thus, weight on July 12th, 4-51 grains; on July 17th, 



