CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 247 



unilaniinar " disc " * of polygonal plate-like cells (Fig. 111). The 

 cells at the edge of the disc (blastocones), however, are not polygonal, 

 having no outer edge (Fig. Ill, lie), but passing over into the 

 remaining mass of formative yolk. In Fig. Ill, a certain difference 

 of size can be perceived in the blastomeres, but this apparently 

 cannot yet be related with certainty to the later form of the embryo. 

 The germ-disc spreads out and increases in size as is evident in 

 Figs. 107-1 11, at first no doubt chiefly at the expense of the formative 

 yolk in contact with it ; only later is the abundant food-yolk utilised 

 as nourishment for the growing- embryo. 



, -.: 



FIG. 112. Germ-disc of Sepia ojftci nails at the commencement of the formation of the 

 germ-layers (after VIALLETON). d, yolk; e, unilaniinar portion of the germ-disc ; 

 ''/, thickened (multilaminar) portion of the germ-disc (area opaca) ; Z, cells in the 



act of separating from the germ-disc. 



After the cleavage-cells have considerably increased in number, the 

 peripheral cells somewhat change in shape ; their free ends narrow 

 and they show a tendency towards becoming detached from the 

 germ-disc (Fig. 112 Z). They finally become rounded off and move 

 away from the disc. They are then found scattered round the latter 

 at the surface of the food -yolk. It should here be mentioned that 



* This so-called disc is actually a more or less arched cap. 



