THE DEVELOPMENT OF THE TADPOLE. 211 



first. Carrying back our observations from the fully-developed 

 structures to the earliest date at which any trace of the limbs can 

 be detected, we find that about a month after the eggs are deposi- 

 ted it is possible to detect the first traces of these appendages. 

 At this period, when the external gills are still active and the alimen- 

 tary canal has not been fully formed, but the proctodeum and lower 

 portion of the intestine can be plainly seen (see Figs, i, 2, and 3, 

 Plate XIII., July number, 1888), there, in the angle formed between 

 the immature tail and the external opening of the alimentary canal, 

 can be seen a few dark-coloured cells of large size, having a very 

 large and conspicuous whitish nucleus, very different to all the sur. 

 rounding cells. These cells are portions of the epidermis, which 

 afterwards divides into the true skin and the ectoderm, but which, at 

 the period named, March 23rd to 25th, is homogeneous, being, how- 

 ever, much lighter in the district pointed out and around the tail 

 than it is on the fore part of the body. A diagram of these cells 

 can be seen in Figs, i, la, Plate XXL, of this part. 



It must be remembered that at this period of growth the 

 development and alteration of all parts is most rapid, and in the 

 course of a few days there is a distinct separation between the 

 skin and the ectoderm, and at the hinder part of the abdomen this 

 occurs to the greatest extent, so that by April ist, when the lower 

 portion of the intestine has extended far beyond the general line 

 of the abdominal curve, there is quite a large space separating the 

 ectoderm from the skin at this point. By the term large space 

 must not be understood a vacuous space. It is, indeed, filled with 

 a most delicate structure, including many multipolar cells, and 

 others, looking like muscle cells, of an elongated lozenge shape, 

 distinctly nucleated, and much attenuated. The upper boundary 

 of this structure consists of the skin muscles, which run all down 

 the abdomen, extend into the tail, and there unite with the great 

 mass of tail muscles which cover the spinal cord and notochord. 



It is within the space thus bounded by true skin and ectoderm 

 that the hind-Umbs begin to grow, forming a small protuberance, 

 consisting, at first, of a few cells, pushing the ectoderm outwards. 

 The external wall is much thicker just over the protuberance than 

 in other parts of the ectoderm, consisting of two layers, the outer 

 being much the darker of the two. This protuberance, once 



