OVIPOSITION AND THE CONSTITUTION OF THE EGG. 



239 



granules, always constitutes by far the greatest mass of the egg. The 

 egg, in shape, is usually oval (Loligo, Eledone, Octopus, Argonauta) 

 or spherical, as in Sepia and the Cephalopods investigated by 

 GBENACHEB (p. 267). The massive food-yolk is completely invested 

 by a comparatively thin layer of the formative protoplasm, which 

 thickens to form a disc-shaped accumulation at the upper or future 

 animal pole of the egg beneath the micropyle. This is the future 

 germ -disc (Fig. 105, A) and is sharply marked off from the food-yolk. 

 In consequence of this peculiarity, and others to be described later, 

 the eggs of the Cephalopoda afford the most perfect examples of 

 mero blast ic cleavage. 



FIG. 106. Diagrammatic longitudinal sections through the egg of Loligo Pealii (after 

 WATASE). B is a median sagittal section ; A , a transverse vertical section at right 

 angles to B. The black line on the periphery of the egg represents the formative 

 protoplasm, while the food- yolk is shaded, d, dorsal side ; v, ventral side ; h, 

 posterior ; vo, anterior ; I, left ; r, right. 



A bilateral symmetry can be recognised not only in the early stages 

 of cleavage but even before cleavage sets in ; this symmetry bears a 

 definite relation to the later development of the embryo. In the oval 

 eggs of Loligo Pealii, it is expressed not only in the form of the egg, 

 but also in the way in which the germ-disc spreads over it (WATASE, 

 No. 50). The eggs of this Cephalopod appear somewhat flattened on 

 one side while, on the opposite side, they are arched (Fig. 106 B). 

 The positions of the anus and the mouth in the embryo correspond 

 to these two sides. At the part which becomes the anterior end (vo), 

 the protoplasm of the germ-disc extends further down towards the 

 equator than on the opposite side (h). The germ -disc, however, 



