218 M. Ussow's Zoologico-Emhryological Investigations. 



duction of the intestinal tract in Sepiola from two opposite 

 invaginations of tlic upper gemi-lamclla. 



As regards the hody-caviti/, I think it will be most correct 

 to give this name to the rather narrow and inconsiderable 

 space which occms between the peripheral layer of tlie dermo- 

 miiscular layer and one or two rows of the intestino-fibrous 

 layer forming the muscular envelope of the intestinal tract. 

 The whole of this completely closed body-cavity is bounded 

 by elongated cells of the dermo-muscular layer forming the 

 peritoneum or peritoneal sac (in which the alimentary appa- 

 ratus, the central organs of the sanguiferous system, and sub- 

 sequently also the generative organs are placed). The inner 

 nutritive vitellus is never enclosed by a special bounding layer, 

 as Kolliker * thinks ; but it lies free in the body-cavity, and 

 the space occupied by it since the commencement of develop- 

 ment represents the segmentation-cavity of the holoplastic 

 ova with total segmentation of many other animals. The 

 respiratory organs, the two branchiai, and the funnel are 

 situated in a special open respiratori/ cavity covered only 

 by the ventral part of the mantle, and lined internally with 

 simple epithelium forming the continuation of tlie upper germ- 

 lamella, penetrating here dm-ing its separation from the ventral 

 surface. 



I have still to notice the period of the appearance of the 

 nervous system and its mode of formation in the Cephalopoda. 

 After a long series of frequently repeated observations relating 

 to this question, and always furnishing the same results, I 

 have been compelled to give up for ever the hope of finding, 

 in the development of the nervous system of the Ceplialoj^oda, 

 any resemblance to its development in the Vertebrata, Tuni- 

 cata, Annulosa, and Mollusca. Whilst even in many species 

 belonging to the types of the Arthropoda and Mollusca some 

 ganglia, at least (as has been proved f), arc undoubtedly de- 

 veloped from the upper germ-lamella, all the ganglia of the 

 Cephalopoda originate from more or less compact thickenings 

 of the middle germ-lamella (dermo-muscular layer), and con- 

 sequently in accordance with the mode of formation of the peri- 

 pheral ganglia in the Vertebrata, which, indeed, has already 

 been partially indicated by MetschnikofF J with regard to 



* Loc. eit. pp. 61, 87, 167. Metschnikoff has justly rejected this view 

 as regards Sepiola. 



t See the abeady cited remarkable memoir by Kowalevsky, M6m. de 

 I'Acad. de St. P^tersb. tome xvi. p. 10, pi. v., and p. 24, pi. vii. ; also 

 B<^)brezky'8 memoir, pi. iii. ; M. (Janin, Warschauer Univer,-<itiitsberichtc, 

 1873, i. ;' and Bericht fiir Anat. und I'hysiol. 1873, p. 300. 



\ Loc. cit. pp. 41, 67. 



