950 SPECIAL PHYSIOLOGY. 



within, the zona pellucida of the Mammalian ova, or upon, or within, 

 the vitelline membrane in other ova. 



In the Mammalia, fertilization occurs in the Fallopian tube, or in 

 the uterus. In Birds and Reptiles, and in the higher Cartilaginous 

 and a few Osseous Fishes, which are ovoviviparous, it takes place as 

 the yolk enters the oviduct, before it receives its coating of albumen. 

 In Amphibia, it happens at the time of deposit of the ova, and in 

 Fishes, with the exception of a few, immediately after. In the Mol- 

 lusca and Molluscoida, Annulosa and Annuloida, and Coelenterata, 

 fertilization occurs within the body, whether the sexes be distinct, or 



Fig. 120. 



Fig. 120. a, spermatozoa of the squirrel, b, spermatozoa of the dog, still inclosed in the sperm-cells. 

 Three spermatozoa are shown free, above. Very highly magnified. (Wagner, Leuckhardt.) 



whether hermaphrodite individuals contain both ovaries and fertilizing 

 organs. Even in the Protozoa, separate nuclear bodies exist, which 

 combine or conjugate, previously to the reproduction of new individ- 

 uals. 



In the Mammalian ovum, it is said that the germinal vesicle ap- 

 proaches one side of the germ-cell, and even has its germinal spot 

 turned in the same direction that is, towards the side directed to the 

 place of rupture in the Graafian follicle. Such a movement would 

 certainly facilitate the access of the fertilizing agent to the germinal 

 vesicle and spot, that is, to the nucleus and nucleolus of this primitive 

 cell. 



In the Frog's spawn, the spermatozoa have been seen in the jelly- 

 like envelopes of the ova, and also within the ovum. In certain Osse- 

 ous Fishes, a minute, funnel-shaped aperture, named the micropyle^ 

 forms, at one period, in the vitelline membrane, and admits the en- 

 trance of the spermatozoa. A micropyle has also been seen in the 

 ova of the Lamellibranchiate Mollusca, of certain Insects, and of some 

 Echinodermata. In these cases, the vitelline membrane is relatively 

 thick. No micropyle has been seen in any of the Vertebrata, except- 

 ing in Osseous Fishes. 



