M. Ussow's Zoohgico-Emhryological Investigations, 109 



interspaces, such as I liave seen in no species investigated by 

 me. The mode of formation of the embryonal cells is also, 

 as follows from tlic preceding statements, quite erroneously 

 described by Kolliker. 



III. The formation of the Germ-lamellce. 



The above-mentioned concluding stage of the process of 

 segmentation {{. e. the appearance of the germinal disk, or the 

 one-layed germ, consisting of the upper germ-lamella^ which 

 appears at the upper pointed part of the nutritive vitellus and 

 covers a twelfth part of it) occurs in most of the Cephalo- 

 poda observed by me on the second day after the commence- 

 ment of development*. The important moment of the 

 appearance of the second germ-lamella falls in the beginning 

 of the third day [Sepia, LoligOj Ommastrephes). The original 

 separation of the second germ-lamella takes place in the fol- 

 lowing manner: — In the middle part of the above-mentioned 

 one-layered ring, situated immediately below the centre of the 

 germ (now very like the area opaca), tlie cells, which are 

 continually undergoing further division in a longitudinal di- 

 rection, begin also to divide gradually in a transverse direc- 

 tion, the division commencing at the loAver periphery and 

 advancing towards the centre. The nucleus of each cell of 

 the one-layered upper germ-lamella becomes elongated ; and 

 at the same time the protoplasm is also elongated, like a drop, 

 downwards ; and then a new cell is constricted off from the 

 mother cell. As the result of this transverse division a second 

 germ-lamella is produced, at first only in the median ring of 

 the germinal disk, but afterwards also in the central part and 

 in the segment-part. At the spots where it has been formed, 

 the germinal disk soon becomes quite opaque, and appears dull 

 white by direct light. 



In the following days (about to the fourth or fifth) the 

 above-described process of growth is continued, and now in all 

 parts of the germinal disk, by which means, 1, the diameter of 

 its still one-layered central part increases considerably ; 2, the 

 middle two- or more-layered thick part [area opaca) spreads 

 more and more towards the inferior pole ; and, 3, the region 

 of the segments dividing up into cell-groups which follows 

 directly on the ring now commences at the equator of the 

 vifellus (and therefore much lower than before). The thick- 



* In Argotuiuia the germinal disk is formed as early as the seventh or 

 eighth hour from the commencement of segmentation. 



