«80 



THE OVUM AND BLASTODERM. 



the first division is effected by a groove or fissure which passes through 

 the thickness of the germinal disc, having probably a direction at right 

 angles to the long axis of the egg. This is crossed by a second fissure so 

 as to divide the disc into four parts near the centre. A third cleavage 

 or fissuring is still of the same radial character, dividing the disc 

 into eight parts or sections ; but this is succeeded by another iu 

 a different direction, which may be named concentric, and which has 

 the effect of separating from the rest those parts of the radial segments 

 of the disc which are next to the germinal centre : a subsequent 

 alternating succession of radial and concentric fissures ends by dividing 

 the whole disc into organised nucleated cells of a similar kind with those 

 by which the whole of the mammiferous yolk becomes covered. A third 

 set of fissures, which may be termed horizontal, must also occur to com- 

 plete the separation of the segmented masses from the subjacent 

 material. In this manner the germinal disc or cicatricula of the bird's 

 egg has already, before the commencement of incul)ation, that is, during 

 its descent through the oviduct and previous to being laid, been con 

 verted by the organising process of segmentation into the layer of cells 

 which constitutes the blastoderm. The cicatricula of the laid egg is 

 therefore of quite a different structure from that of the ovarian ovum, 

 though occupying the same place and presenting much the same ap- 

 pearance to the unassisted eye. This layer appears to be double from 

 the first in the bird's egg, or to consist of two strata of cells, differing 

 somewhat in their character. 



Fi' VJj. 



Fig. '195. — Cicatricula of the 

 Bird's Egg. 



A, diagrammatic section 

 tlirough the cicatricula of a 

 newly laid egg ; a, vitelline 

 membrane ; b, segmented germ 

 disc ; c, below this the germ 

 cavity ; d, the yolk cavity with- 

 in the white yolk ; c, c, the 

 yellow yolk substance. 



B, view from above of the 

 cicatricular or germ disc of a 

 newly laid impreguatetl egg in 

 whicli segmentation has been 

 complete. The opaque area is 

 seen surrounding the central 

 transparent area. 



C, cicatricula of an unim- 

 pregnated hen's egg, showing 

 the vacuolar .structure produced 

 by incomplete segmentation. 



In this process there is much which is obscure and still imperfectly 

 known, and much to excite our curiosity. The source of the first 

 segment nucleus has not been discovered, nor is it known whether or 

 in what way it may be related to the dis]iersed contents of the germinal 

 vesicle or to its macula, and we are equally in the dark as to what may 

 be the influence of the spermatic element upon the germ, and what the 

 forces by which the cleavage and the formation of the multiplying 

 spheres are brought about. 



Contractile and other movements in the germ. — With respect to the last 



