676 



THE OVUM AND BLASTODERM. 



oviduct, and it is now ascertained that the spermatozoa not only adhere 

 in nnmbers to the external surface of the ovum, but actually penetrate 

 throu2;h the zona, so as to come in contact with and possibly also com- 

 bine with the substance of the yolk (see figs. 491, 492, and 493). We 

 are, ho^Yever, entirely ignorant of the nature of the operation of the 

 spermatozoa upon the substance of the germ. The shrivelled remains 

 of these particles are seen for days adherent to the ova, and even in the 

 substance of the germ, and though doubtless they at last disappear, it 

 has not been determined whether this is by combination of their sub- 

 stance with that of the germ or in what other way the mutual or 

 reciprocal action of the male and female generative elements may 

 take place. 



The fact remains as one of the most remarkable in the whole range 

 of biological phenomena, that by the contact of an inappreciable amount 

 of the male product with the germinal material of the ovum, the latter 

 passes from an apparently inert condition into one of genetic activity, 



Fig. 491. 



Fig. 491. — Ovum of the Rabbit from the 

 Falloi'Ian Tube, twelve hours after Im- 

 pregnation (from Bisclioff). =f'^ 



A few gvaimlar cells adhere to the outer surface 

 of the zona, in which and in the zona itself sper- 

 matozoa are seen ; «, zona ; b, t'vvo hyaline globules 

 witliin the cavity left by the shrinking of the yolk. 



the ultimate result of which is the ac- 

 complishment of a series of the most com- 

 plicated phenomena of organic formation 

 and growth, giving rise to a new being, 

 which, while it may be of either sex, 

 repeats in all respects the characters of the species, and may inherit in 

 a greater or less degree the minutest peculiarities, whether structural 

 or functional, of either or of both its parents. 



There are two changes following impregnation which have been observed in 

 the mammars ovum, and which are deserving of notice, though their import is not 

 yet known. One of these changes consists in a certain contraction or dimiau- 

 tion in the size, and an increase in the apparent compactness or firmness of the 

 mass of the yolk, so that a larger space than before, filled with clear fluid, 

 comes to intervene between the yolk and the suiTOunding zona. The other 

 change referred to is the appearance in this space of one, or most frequently 

 two. and occasionally of three, clear or hyaline spherules, which are easily dis- 

 tinguished from the surrounding fluid by their peculiar highly refracting out- 

 line (Quatrefages, Ed. Van Beneden, Bischoff). These spherules are of variable 

 size, but generally their diameter is from one-tenth to one-fifteenth of that of 

 the mammal's ovum (fig. 4yi, />, and 492. A). They are perfectly hyaline and 

 homogeneous and do not appear to possess any external envelope. They remata 

 visible for some days during the early j)hases of yolk-segmentation, about to be 

 described, and hence by some they have been named seginentation globules. 

 Their source and destination, however, are entnely unknown. 



Segmentation of the yolk or germ. — After the disappearance of 

 the germinal vesicle the germinal part of the yolk constitutes for a 

 time a non-nucleated mass of protoplasm ; and if then subjected to the 

 influence of fecundation it undergoes the change of segmentation, which 

 results in the conversion of the germ or germinal part of the yolk into 



