122 SCIENCE PROGRESS. 



of contributing to the coloration of the. animal. In its 

 final stage it is rendered so complex by the number of 

 accessory nervous and muscular structures which become 

 connected with it, that the question may well be asked by 

 what phylogenetic steps so elaborate an organ has been 

 evolved. As an answer to this question I would invite 

 comparison between the developing chromatophore of 

 Argonauta as described by Joubin, and the unicellular 

 " purple-glands ' : of the mantle-edge of Aplysia as de- 

 scribed by Blochmann (20). In Aplysia limacina each 

 of the purple gland-cells is at first a part of the ectodermal 

 epithelium ; it enlarges and sinks beneath the epithelium, 

 retaining only a narrow, neck-like prolongation to the 

 surface ; the whole of the cell then sinks deeper within 

 the mesoderm, but its connection with the exterior is 

 retained by the simultaneous development of a multicellular 

 duct of ectoderm cells formed by invagination of the 

 epithelium. Each gland-cell also becomes surrounded by 

 connective tissue cells and muscle-cells, by the contraction 

 of which the pigmented secretion of the gland is forced to 

 the exterior. The striking histological resemblances be- 

 tween these unicellular glands of Aplysia and the chromato- 

 phores of Cephalopods did not escape Blochmann's notice ; 

 but now that Joubin has furnished us with embryological 

 proof of the ectodermal origin of the chromatophore, and 

 has figured an ectodermal invagination which corresponds 

 in all respects to the multicellular duct of the gland-cell in 

 Aplysia, it becomes possible to suggest distinct homologies 

 between the two sets of structures. The developmental 

 stages of the chromatophore in Argonauta appear to me to 

 be largely recapitulative of its past history ; and I would 

 suggest that the chromatophores of Cephalopods have been 

 evolved by the progressive modification of some such com- 

 plex ectodermal gland as that of the mantle-edge of Aplysia. 

 The only serious difference between the two structures is 

 that in the Cephalopod the peripheral muscle-cells expand 

 the pigment-cell, whereas in Aplysia they contract it. 

 Kliemensiewicz, however, maintains that in addition to 

 the radial muscle-fibres there is a distinct cellular envelope 



