290 GENERATION. 



in this way that the ovum is fecundated. In the osseous fishes, 

 there seems to be no question with regard to the existence of 

 numerous pores in the vitelline membrane ; but these are not 

 so easily demonstrated in the ova of mammals. Many years 

 ago, Barry described and figured a large opening, or micro- 

 pyle, in the ovum of the rabbit, in which he thought he dis- 

 tinguished, on one occasion, a spermatozoid half-way through 

 the membrane ; l but this was probably an accidental rupture. 

 A so-called micropyle has been described by Keber, in certain 

 mollusks, 2 the existence of which is admitted by Coste, though 

 he doubts the observation in which Keber described a sper- 

 matozoid, with the tail detached, in the act of penetrating 

 the membrane. Coste himself states that the micropyle is 

 seen with distinctness in the osseous fishes, as the salmon, 

 trout, etc. " At the bottom of an umbilicus, visible with a 

 simple loop, the vitelline membrane is pierced with a micro- 

 scopic orifice, clearly defined, provided internally with a little 

 valve." J Admitting the existence of a micropyle and pores 

 in the vitelline membrane, in fishes and mollusks, it is certain 

 that openings are very much more indistinct, if they can be 

 seen at all, in the ova of mammals ; still, the fact of the ac- 

 tual penetration of spermatozoids almost of necessity presup- 

 poses the presence of orifices. We have often thought, in 

 studying this subject, that it must be difficult, examining a 

 perfectly transparent and homogeneous membrane in water, 

 which would fill up all pores, to distinguish any openings, 

 and have been disposed to admit their presence, mainly be- 

 cause the spermatozoids are known to pass through. The 

 idea of their existence in mammals certainly receives sup- 

 port from analogy with the lower orders of animals. 



The vitellus, called the principal yolk or the formative 



1 BARRY, Researches in Embryology. Philosophical Transactions, London, 

 1839, Part ii., p. 533. 



2 KEBER, Ueber den Eintritt der Samenzellen in das Ei, Konigsberg, 1853, S. 

 18, et seq. 



3 COSTE, Developpement des corps organises, Paris, 1859, tome ii., p. 106. 



