EMBRYOLOGY. 97 



sex of an embryo is undistinguishable until a 

 comparatively late period in foetal life. Further, 

 there is absolutely no means by which a com- 

 parative anatomist can distinguish between the 

 foetus of a man, beast, bird, or lizard, until the 

 limbs have already passed through their earliest 

 stage of development ; * and the first rudiments 

 of the legs and wings of a bird are of similar 

 shapes.*} 1 



In its earliest stage, it is impossible to tell 

 whether an ovum is that of an animal or a plant ; 

 for even the ovum of a mammal presents at one 

 period an extremely close resemblance to Volvox 

 Globator, one of the lowest microscopic plants. 

 Next, when the ovum is a little further de- 

 veloped, so as to be recognised as that of an 

 animal, its sub-kingdom is still doubtful ; when 

 it manifests itself as a Vertebrate, its Class is 

 still indeterminate ; and then the characters 

 peculiar to its Order, Family, genus, species, and 

 individuality appear in succession, though in 

 some cases the specific characters appear before 

 the generic.J 



* Von Baer, quoted by Darwin, " Origin of Species," p. 470. 

 t H. Spencer, " First Principles," p. 336. 

 \ Carpenter, op. tit. pp. 95, 96. 



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