AND THE LOWER ANIMALS. 49 



mammals, extends beyond the legs, which as yet are 

 quite like the arms. At this period the human embryo 

 is not by any means a bad representation of a certain 

 species of seal. In order to be converted into the 

 foetal limbs and arms, these organs must undergo 

 considerable modifications, and be increased in size. 



In the construction of a new being there is some- 

 thing more than the creation and successive fashioning 

 of organs which will eventually assume their adult 

 characters. In the higher animals Nature always 

 supplies those wants which the progress of develop- 

 ment brings with it, and as these requirements are 

 not unfrequently temporary, the organs which cor- 

 respond to them are often transitional. Hence we 

 often find her simultaneously building up one set of 

 structures and destroying another. 



A careful study of the circulatory apparatus would 

 supply us with several curious examples, but we 

 undoubtedly meet with the most remarkable instances 

 in the organs of secretion. Among the manifold por- 

 tions of the frame we find certain structures, which 

 have been called after the great anatomist who first 

 examined them carefully, the Wolffian bodies. These 

 bodies remind us forcibly of the kidneys, at least as 

 to structure, and appear to have a similar office to 

 fulfil. They are seen at a very early period of em- 

 bryonic life, and soon extend from one end of the 

 body to the other, lying on either side of the intestinal 

 canal. As soon as the true kidneys present them- 

 selves, these Wolffian organs disappear, so much so, 

 that in a few adult mammals we can find but very 

 questionable traces of them, and the detection even 

 of these is a matter of some difficulty. 



E 



