1 20 Darzvin, and after Darwin, 



is, that the embryonic phases of the higher form 

 resemble the corresponding phases of the lower forms. 

 Thus, for example, it would be wrong to suppose 

 that at any stage of his development a man resembles 

 a jelly-fish. What he does resemble at an early 

 stage of his development is the essential or ground- 

 plan of the jelly-fish, which that animal presents in 

 its embryonic condition, or before it begins to assume 

 its more specialized characters fitting it for its own 

 particular sphere of life. The similarities, therefore, 

 which it is the function of comparative embryology 

 to reveal are the similarities of type or morphological 

 plan : not similarities of specific detail. Specific details 

 may havebeen added to this, that and the other species 

 for their own special requirements, after they had seve- 

 rally branched off from the common ancestral stem ; 

 and so could not be expected to recur in the life-history 

 of an independent specific branch. The comparison 

 therefore must be a comparison of embryo with 

 embryo ; not of embryos with adult forms. 



In order to give a general idea of the results thus 

 far yielded by a study of comparative embryology in 

 the present connexion, I will devote the rest of this 

 chapter to giving an outline sketch of the most im- 

 portant and best established of these results. 



Histologically the ovum, or egg-cell, is nearly 

 identical in all animals, whether vertebrate or in- 

 vertebrate. Considered as a cell it is of large size, 

 but actually it is not more than y^y, and may be less 

 than 77^0 of ^'"i inch in diameter. In man, as in most 

 mammals, it is about ^\^. It is a more or less spherical 

 body, presenting a thin transparent envelope, called 



