148 REPRODUCTION IN PLANTS AND ANIMALS. 



served on tlie same individual; wliereas, if the eggs of 

 birds are employed, whicli are opaque, an egg must be 

 sacrificed for eacli observation.* 



The only essential diiference between the egg_ of mam- 

 mifers, and the human species in particular, compared with 

 the eggs of oviparous animals, is in its excessive minute- 

 ness. When fully developed, its diameter hardly amounts 

 to the tenth of a line. It consists in a very small vitelline 

 mass, enveloped in a thick transparent membrane, which 

 some authors have called the chorion, whilst others have 

 considered it as representing the albuminous part. These 

 ovules are each placed in the vesicles of Graaf, which are 

 small bladder-like bodies, contained within the ovarium, 

 the remainder of the vesicle being filled with albuminous 

 fluid. At the time corresponding to the epoch of rut or 

 menstruation, the Graafian vesicles are ruptured and the 

 ovule escapes, is immediately seized by the fimbriated pro- 

 cesses of the fallopian tube, and thence conveyed along the 

 tube as far as the uterus. If, in its course, the egg encoun- 

 ters the seminal matter of the male, fecundation is effected. 

 It then becomes attached to a point of the matrix, where it 

 passes through all its developments till the moment when 

 it arrives at maturity. It then comes out to the exterior 

 and its extra-uterine life commences. If, on the contrary, 

 the egg is not fecundated, it usually dies within a few days, 

 and is lost to reproduction. 



The seminal matter which fertilizes the ovule, is in 

 ' animals ordinarily a compact fluid, which contains granu- 

 lations more or less abundant, and animated corpuscles 

 called spermatozoa, or spermatic animalcules of a very 

 variable form. The spermatozoa have a body some- 

 times round, and occasionally pyriform or cylindrical, 



* Louis Agassiz. 



