210 ^[. Ussow's Zoohgico-EmhryologicaJ Investigations. 



verted into a tube in the manner above mentioned, consist of 

 a single layer of cells * of the upper germ-lamella ; whilst in 

 the oval fold (rudiment of tlie nuvntle), besides the elongated 

 cylindrical colls situated at its surface, there arc also two layers 

 of cells of the middle germ-lamella. The first of these layers 

 (dermo-museular layer), constantly increasing with the deve- 

 lopment of the fold, becomes more than one-layered under its 

 margins t, and therefore also thicker; and this thickening is 

 the immediate cause of the evcrsion of the fold over the blasto- 

 derm and its constriction on the ventral side. 



Besides the above-mentioned organs, the rudiments of ej/e- 

 ovals and of the buccal orifice make their appearance at this 

 time. The buccal orifice, which can only be recognized with 

 some trouble from without, appears in longitudinal sections of 

 this stage as a very shallow depression of the upper germ- 

 lamella. The rudiments of the eyes, which lie symtnetrically 

 on the sides of the dorsal surface, are developed chiefly from 

 the elongated cells of the upper germ-lamella, the single 

 series of which forms a longish oval convexity J above the 

 blastoderm. 



The Cephalopod embryo, freed from the nutritive vitellus 

 in the manner already described (see p. 100, notef), in this 

 first stage of the production of the organs has the form of a 

 convex disk, or rather of a hollow hemisphere, composed of 

 more than one layer and more or less thickened in many 

 places. The earliest and most considerable thickening corre- 

 sponds to the scutiform mantle-rudiment, pointed on the dorsal 

 surface, and curvilinearly bounded on the ventral side by 

 the above-described rhomboidal groove, which in transverse 



* The cylindrical cells lining the bottom of the groove are rather tall, 

 whilst the layer which covers the groove and subsequent!}' grows together 

 consists of small flat cells. Some agreement in the production of this 

 groove and that of the intestino-glandular [epithelial] layer of certain 

 animals (e. y. the Arthropoda), and the great resemblance of its cells 

 underlying the upper germ-lamella to those of that layer, at first led me 

 astray, and made me think that perhaps in the Cephalopoda also a portion 

 of the intestinal tract is formed as in the Crustacea (see the remarkable 

 Russian memoir of Bobrezky, " On the development of AMacm and 

 Pal<emon''). It was only a long series of repeated observations that 

 convinced me of my original en-or. 



t The part of the dermo-muscular layer which is situated between the 

 groove and the surface of the mantle becomes converted (in the third 

 period) into the cutii with its muscuhir and fibrous layer. 



I This mode of development of the primitive eye- ovals, which are 

 soon covered by a second fold of tlie upper lamella and then gradually 

 begin to .sink, ha.s been quite correctly observed by Metschnikofrin Septula 

 (/. c. pp. 4'3-49). As regards the other Cephalopoda, it is confirmed by 

 my investigations ; and consequently Kcillikor'.s (/. c. p. 09) and Ilensen's 

 (Zeitschr. uir wi.ss. Zool. Bd. w. p. 183) statements prove to be erroneous. 



