120 Darwin, 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 
zts 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 ;3,, and may be less 
than zt, of an inch in diameter. In man as in most 
mammals, it isabout z35. It isa more or less spherical 
body, presenting a thin transparent envelope, called 
