M. Uisaow's Zoohgko-Emhryological Investigations. Ill 



at the time wlieu the nutritive vitellus is entirely surrounded* 

 at the interior pole by the cells of the upper germ-lamella 

 formed by terminal division of the segments, and by the 

 upper layer of elongated fusiform cells of the second germ- 

 lamcUa. 



On the seventh and eighth days the germ enclosing the 

 nutritive vitellus gradually changes its form from oval to 

 perfectly spherical. In Loligo^ Sepioluy and Ommastrephes the 

 surfaces of most of the cells of the upper germ-lamella (sphe- 

 rical embryo) (those on the part where the eye-ovals will be 

 formed and some others excepted) become covered with cilia, 

 which, in the species above enumerated, cause the rotation of 

 the embryo by their continual movement. In Sepia and 

 Argonauia the embryo does not rotate, either in this or the 

 following stage of development. The period of formation of 

 the blastoderm (including the process of segmentation) lasts 

 from four (Argonaufa) to nine {Loligo, Seviola) and more days 

 i:} Sepia). 



Thus at the commencement of the rotation, with which the 

 second period of development (that of the production of the 

 organs) begins, the germ covers the whole of the nutritive 

 vitellus, and consists of two gerra-lamellte here and there 

 composed of several layers, namely : — 1. The blastodenn or 

 upper germ -lamella (Ilornllatt). The thickness of this lamella, 

 which is still one-layered, increases somewhat as we approach 

 the upper pole of the nutritive vitellus t, and, indeed, at the 

 point where the oval fold covering the rhomboidal part of 

 the germ, situated on the dorsal sm-face of the embryo, is 

 formed. The rhomboidal centre of the germ, which was 

 at first round, and the oval, broader or narrower annular fold 

 originate from the considerably grown central part of the ger- 

 minal disk, situated at the acute pole and bordering upon the 

 so-called area opaca ; but this part itself has originated from 

 the fourteen primitive segmentation- spheres, which rajjidly 

 increased in number and appeared at different times. The 

 middle portion of the germ, which now covers nearly half 

 the surface of the nutritive vitellus (from the margin of 

 the above-mentioned fold to the equator) and attains its 

 greatest breadth upon the dorsal surface, represents the con- 

 siderably widened middle ring of the germinal disk, which 

 originated from the multiplication of the cells chietiy con- 

 stricted off from the segments by the meridional furrow. Here 



* In Loligo, Sepiola, and Argonauta ; in Sepia the blastoderm, as already 

 remarked, only close-; in the second period of development. 



t By transverse division of its cells, which become cylindrical and 

 generailv contain two sharply defined nuclei. 



