702 OF REPRODUCTION. 



as is shown in Fig. 11, Avhere concentric rings of new cells are seen in each 

 of the parent vesicles. Even in the most advanced of these secondary cells, 

 another generation may be seen, and these are developed upon the same plan 

 with those of the Germinal Vesicle : thus in Fig. 12, the pellucid centre of 

 the original nucleus of the parent disc is seen at a, and is surrounded by seve- 

 ral concentric rings of cells, increasing in size from within outwards ; and at 

 b is represented the condition of the outer and older cells, in which the same 

 process is undergoing repetition. (Although the figure only represents one 

 secondary cell as in the act of producing others, the others of the same age 

 are alike engaged in the process of multiplication.) The foregoing history is 

 equally applicable to the cells, from which the Embryo subsequently origi- 

 nates ; and it is probably the general mode in which the process takes place. 



917. At the time when the interior of the Germinal Vesicle is being pre- 

 pared for the reception of the fecundating influence, the portion of the Zona 

 Pellucida against which it lies becomes attenuated ; and a chink then forms in 

 it, just above what was the pellucid centre of the Germinal Spot. Through 

 this chink, the Spermatozoon can reach the Germinal Vesicle; and that it does 

 so, we are now entitled to affirm, not only from analogy, but also from actual 

 observation ( 900). What is the nature of the influence communicated by it 

 is less certain; but from the known character of the process of fecundation in 

 Plants, we shall have little difficulty in concluding, that it deposits in the 

 Germinal Vesicle the rudiments of the first cells, which are subsequently to 

 be developed into the Embryonic structure. It is certain that none of the 

 cells previously contained in the Germinal Vesicle subsequently form part of 

 it ; in fact, they all liquefy after a time, and disappear entirely. But in the 

 previously pellucid centre of what Avas the Germinal Spot, two new cells are 

 seen after fecundation ; these enlarge at the expense of the rest; and from 

 them, all the permanent structures originate. This pair of cells is seen at a, 

 Figs. 13 and 14; in the former some of the cells of the Germinal Vesicle are 

 still left ; in the latter, they have been all absorbed. The Germinal Vesicle 

 returns after fecundation to the centre of the Yolk, being at first entirely con- 

 cealed by its discs (Fig. 11); and the cleft in the Zona Pellucida soon closes, 

 so as to be no longer distinguishable. The two new cells and the other con- 

 tents of the Germinal Vesicle, undergo such a rapid increase in size, that they 

 soon fill the whole interior of the Zona Pellucida ; and the cells of the Yolk 

 being reduced by the pressure into a liquid form, their elements are absorbed 

 by the new cells of the Embryonic structure. This, at least, is the case in 

 the Mammalia; among which the Yolk performs but a very subordinate part, 

 having only to serve for the development of the Embryo during a very brief 

 period. In each of the two primary Germ-cells (as they may be called) a 

 series of changes takes place, exactly conformable to that already described as 

 occurring in the Germinal Vesicle ; that is to say, a ring of new cells origi- 

 nates in the margin of its nucleus, this increases in size, and is pushed out- 

 wards by another ring nearer the centre, this again by another, and so on, 

 and at last, two cells appear in the pellucid central space, which are developed 

 at the expense of all the rest, and are to be regarded as the real permanent 

 offspring of the parent. These changes may be seen in progress in Figs. 13 

 and 14; in the former, the original cells of the Germinal Vesicle have not 

 quite disappeared, although their liquefaction is in progress ; in the latter, no 

 vestige of them is left, the whole cavity being occupied by the twin-cells. 



918. These changes commence during the passage of the Ovum along the 

 Fallopian tube; and during its transit to the Uterus, it acquires a sort of gela- 

 tinous envelope, which is inclosed in a membrane of fibrous texture, termed 

 the Chorion. The gelatinous envelope is probably of an albuminous nature 

 in reality, corresponding with the white of the Bird's egg; whilst the fibrous 



