310 CEPHALOPODA. 



size and by the early deposit of pigment in their protoplasm (Gieod, 

 No. 1 3). At a later stage, they are covered by a thick envelope ; 

 the cells in the neighbourhood stretch out into spindles and become 

 connected with the chromatophoi^al cells. In this way arises the 

 well-known appearance of the (contractile) fibre-bundles connected 

 radially with the chromatophoral cells. The change of shape in the 

 pigment-cells which is accompanied by change of colour was usually 

 attributed to the contractility of these bundles, i.e., they have been 

 regarded as muscle-fibres, while some authors have ascribed a capacity 

 for contraction to the pigment-cells themselves, the radial fibres being- 

 considered as merely connective tissue which, it was assumed, held 

 the actual chromatophores in position (Girod). 



Another account of the origin of the chromatophores has recently 

 been given (Joubin, No. 23). According to this view, the ectoderm- 

 cells, which are especially distinguished by their size, sink inwards 

 through a funnel-like depression. In the large cell at the base of 

 the depression, the protoplasm becomes differentiated and, later, pig- 

 ment appears. The cell then loses its connection with the ectoderm. 

 A number of mesoderm-cells which could be seen even earlier regularly 

 arranged below it, and which soon multiply still further, yield the 

 radial fibres. The chromatophores would thus be due to the com- 

 bined action of the outer and the middle germ-layers (Joubin). 



LITERATURE. 



1. Adams, A. Mollusea. The Zoology of the Voyage of H.M.S. 



Samarang. London, 1848. 

 •J. Appelloff, A. Om skalets bildning hos Sepia officinalis. Ofver- 



sigt kongl. Vet.-Akad. Forlandl. Stockholm. No. 7. 1887. 



3. Beneden, P. J. van. Recherches sur rembryogenie des 



Sepioles. Nouv. mem. Acad. roy. sci. Bruxelles. Tom. xiv. 

 1841. 



4. Bobretzky, N. On the Development of the Cephalopoda. 



(Russian.) Izvyest. imp. Obshch. lyubit. estestv. Antrop. i 

 Ethnog. Moscow. Tom. xxiv. 1877. 



5. Brock, J. Ueber die <Teschlechtsorgane der Cephalopoden. 



Zeitschr. f. tviss. Zool. Rd. xxxii. 1879. 



6. Bbock, J. Versuch einer Phylogenie der dibranchiaten Cepha- 



lopoden. Morphol. Jahrb. Rd. vi. 1880. 



