600 OF REPRODUCTION. 



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 several 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 pro- 

 ducing 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 originates ; and it is probably the general 

 mode in which the process takes place. 



746. At the time when the interior of the Germinal Vesicle is being prepared 

 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 ( 733). 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 was 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 , 

 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 Embrj^onic 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 

 originates in the margin of its nucleus, this increases in size, and is pushed 

 outwards 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. 



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

 Fallopian tube ; and during its transit to the Uterus, it receives a very import- 

 ant addition, that of the Chorion. According to Dr. Barry, the Choriori 

 originates as a layer of cells, the remains of which may be frequently seen, 

 when the membrane is otherwise complete (as in Fig. 14, Plate I.) ; and the 

 space between the Chorion and the Zona Pellucida is occupied by a quantity 

 of gelatinous-looking fluid, which he supposes to be drawn within the former 



