THE GENERAL CONSTITUTION OF LIVING BEINGS. 77 



the starting-point of the formation and development of 

 the embryo, that is, in the ovule. We shall there point 

 out these three modes at work. 



The ovule is a little globule, from one to two tenths of 

 a millimetre in diameter, that is, as large as a grain of 

 sand that can but just be seen. It is made up of an en- 

 veloping sphere, called the vitelline membrane, in which 

 is found a semi-liquid gelatinous matter, to which the 

 name of vitellus 1 is given. The vitellus in its turn presents 

 a sort of nucleus which is the germinating vesicle, or that 

 of Purkinje. Thus the ovule offers to view at the outset 

 the marks of a true cell, but it gains, while developing, a 

 structure and dimensions that soon distinguish it from one, 

 and make of it a special organ. When it has reached the 

 period of maturity, the germinating vesicle disappears, and 

 its substance mingles with that of the vitellus. At the 

 same time the latter shrinks upon itself, and contracts. 

 There comes between it and the wall of the vitelline mem- 

 brane a space which fills up with a clear liquid. It is at 

 this moment that the phenomenon of fecundation occurs, 

 which is owing to the penetration of the spermatozoa, 

 which make their way into the newly-formed space that we 

 have mentioned. Then the vitellus loses shape, and for sev- 

 eral minutes goes through a series of very varied gyratory 

 motions, which Robin has studied. Simultaneously the 

 spermatozoa which are, as Robin has proved, true ana- 

 tomical elements proceeding from male ovules, analogous 

 to the embryonic cells of the female ovules dissolve, 

 and thus mingle the substance of one parent with that 

 of the other, which they impregnate. There is then re- 

 marked a very curious fact, also discovered and studied 



1 Birds' eggs contain their ovule in the centre, the development of 

 which, instead of taking place by means of materials provided directly 

 by the mother, is made at the expense of materials contained in the eg;.:, 

 that is, the white and the yelk. 



