110 M. Ussow's Zoologko-Emhrifological htvestigations. 



eiied inner layer of the area opaca^ which consists of rounded, 

 scattered, spontaneously dividing cells* (of the second germ- 

 lamella), forms, at the boundary of the central part of the 

 germinal disk, a wall wliich penetrates more or less into the 

 nutritive vitellus. In consequence of this pressure, the nutri- 

 tive vitellus on its part ])enetrates into the slightly rising 

 central part of the germinal disk (like the " Dotterpfroi)fc " of 

 the frog's egg). At the same time a very narrow second ring 

 is formed from the cells separated by constriction from the 

 segments ; this lies between the first ring and the segments. 

 On the sixth and seventh days this new ring exactly sur- 

 rounds the equator of the vitellus. Its four- or five-angled 

 cells, which are rather large, lie in consecutive series. Indeed, 

 in general, all the cells, both of the inner and outer germ- 

 lamella, arrange tliemselves in such consecutive series ; the 

 latter, during their division (constriction), when they are for a 

 short time free, move upon the surface of the nutritive vitel- 

 lus by means of their contractile protoplasm and longer or 

 shorter pseudopodia. 



At the end of the seventh day the cells of the central coni- 

 cal part of the upper germ-lamella multiply very rapidly by 

 longitudinal division (Sepiola, Loligo, Argonauta). By this 

 means is produced a thickening, which, however, by no means 

 occupies the whole central part of the germinal disk, but only 

 forms at its margin an oval fold, which, spreading in the polar 

 direction, begins gradually to conceal the central part. Simul- 

 taneously with the formation of this fold, the part of the 

 germinal disk circumscribed by the fold sinks a little and 

 forms a fun'ow broader and deeper in the middle, having the 

 shape of an extended rhomboid. In the rhomboid the ger- 

 minal disk consists of a single layer of cells of the upper germ- 

 lamella. But beneath the oval fold the cell-layer of the 

 second germ-lamella which is there thickened begins to 

 double itself by transverse division, and thus forms two 

 layers — the upper the dermo-muscular layer, and the lower 

 the intestino-Jihrous layer. These two layers may be most 

 clearly observed at the boundary of the former area opaca 

 and the central part of the germinal disk, and, indeed, on 

 the future ventral surface of the embryo, whilst at first they 

 gradually coalesce towards both the equator and the pole, so 

 as not to be distinguishable. The further splitting of the 

 second germ-lamella into two supeqjosed layers takes place 



* The cells divide both in a longitudinal and transverse direction, by 

 which means their layer becomes tliickcr and broader towards the obtuse 

 pole. 



