234 HOMOLOGICAL EELATIONS 01 THE 



free within the ovum, readily coalesce. Then follows the 

 cleavage of the yolk, and the formation of a wall on its ex- 

 terior — the blastodermic vesicle. 



§ 2. If we turn now to the homological relations of the 

 germinal element in the two kingdoms of organic nature, 

 and take for our starting point the protoplasmic corpus- 

 cules of the embryo-sac of the plant, and the contents of 

 the germinal vesicle of the animal ovum, we may consider 

 the '' formative yolk" of the latter as the equivalent of the 

 vegetable endosperm, and its primordial wall (zona pellu- 

 clda) as representing the embryo-sac of the ovule of the 

 Phanerogamia or the archegonium of the Cryptogamia. 

 Among the adventitious parts or those met with only occa- 

 sionally, we might compare the "nutritive yolk" of the bird's 

 ovum, which is really derived from the contents of the ovisac, 

 to the nucleus of the ovule or to the perisperm of the seed ; 

 and the albumen and shell of the egg (equivalent perhaps to 

 the tunica granulosa and shaggy chorion of Mammalia) to 

 the seed coats, arillus, &c. The ovary of the animal would 

 represent the ovary or germ en of the plant with its loculi. 



In animals, on account of the germ not being generally 

 — like that of the plant — impregnated in situ, that is to 

 say, not coming in contact with the spermatic particles 

 till it has escaped from the ovary, we have no aperture cor- 

 responding to the micropyle of the seed. The term " mi- 

 cropyle" has indeed come into use in connection with the 

 animal ovum, but the aperture so called does not corres- 

 pond to the micropyle of the seed, and is nowhere repre- 

 sented in the majority of j)lants, for though the canal of 

 the archegonium of the Cryptogamia may fairly be taken to 

 represent the micropyle of the Qgg in Fishes, Insects, Echino- 

 dermata, &c., there exists no such opening in the embryo- 

 sac of the phanerogamic plant, the manner of fusion of the 

 reproductive elements being as obscure as in the imperforate 

 ova of the hioher Vertebrata. 



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