SEGMENTATION OF EGGS. 



643 



to a single sex. At times, however, males and females come 

 out in equal numbers, and this is to be explained on the grounds 

 of the host containing more than one chain of embryos, since 

 the parasite sometimes deposits more than one egg within that 

 of its host. The phenomenon of polyembryony has a wide 

 biological significance on account of its bearing on the problem of 

 sex. Similar processes occur among the Polyzoa (Vol. I., p. 563). 

 Within the vitelline membrane which is secreted by the egg 

 itself lies a layer of protoplasm, the periplasm, comparatively free 

 from yolk ; this surrounds a central mass in which the protoplasm 



a 



FIG. 403. Embryonic development of Calepteryx virgo (after A. Brandt), a commencing 

 involution of the ventral plate. The blastoderm was at first one-layered and thickened at 

 the poles. G edge of ventral plate, b Later stage of the involution, c The embryonic 

 membranes are developed ; Lp parietal (serosa) ; Lv visceral (amnion) layer of the latter. 

 d The appendages have sprouted out on the ventral plate. A antenna ; Md mandible ; 

 MX' first maxilla ; MX second maxilla (labium or lower lip). Then follow three pairs of 

 legs, e Byersion of the embryo which is protruded from the sheath of the visceral layer. 

 / Completion of the inversion ; the hind end of the body is free ; the yolk sac is on the 

 dorsal surface. 



is scanty and crowded with food-yolk (Fig. 401 ). The nucelus lies 

 in the central portion, but approaches the surface when mature, 

 and then it gives off successively two polar bodies and, after re- 

 ceiving the male pronucleus, passes back as the cleavage nucleus 

 again towards the centre of the egg. It then divides, and, when 

 the products of division have reached a certain number, most of 

 them approach the periphery and around each a certain cell- 

 territory becomes marked off in the periplasm. Thus an 



