CELLULAR DEGENERATION AND FORMATION OF PIGMENT. 119 



A study of the structure and mode of development of the 

 pigment granules would seem to show that they gradually 

 arise in the protoplasm of the hydroid cell from the limit of 

 visibility until a colourless corpuscle of considerable size is 

 formed (text-fig. 6, k,/. ) . Subsequently the corpuscle darkens 

 by a further decomposition of the proteid, and the actual 

 colouring matter arises. This black or dark brown substance 

 may be especially concentrated in small lumps {g.) in the 

 corpuscle, or it may form a crescent or ring around the 

 peripheiy of the corpuscle {h., Ji.). Instead of forming a disc 

 the body may be rod-like and the black substance may be 

 concentrated in the form of two black parallel streaks 

 (o., ?'., s.). Many may be tempted to believe that this is 

 really some stage of fission of a simple schizophyte ; but, as 

 stated above, it is very doubtful whether these discs or rodlets 

 can be rightly regarded as parasitic organisms foreign to 

 the tissues of the hydi-oid. 



In the ectoderm a single cell, or a cluster of several, 

 produces the pigment granules in a similar manner to that 

 described in Lytocarpus filamentosus and Thuiaria 

 tubulif ormis. The granules appear in the protoplasm 

 surrounded by a halo of unstainable substance which is 

 apparently mucilaginous (6g. k). The mucilage coat, how- 

 ever, is more evanescent than in the other species, and in 

 granules that have become isolated from the protoplasm it 

 does not appear to remain attached to the granule as a 

 definite layer, and consequently zooglea-like masses are not 

 obvious. Cavities form early in the cell or cluster of cells, 

 and these tend to run into one central cavity which is 

 densely surrounded by the pigment granules. The nucleus 

 or nuclei disappear. There is usually formed an oval sac 

 measuring some 9*2 ju in the long axis (figs, b, e, p and g). 



In the endoderm the formation of pigment granules is 

 much less conspicuous; a few granules may be formed in a 

 cell or in an oval area formed by several cells or portions of 

 cells. Veiy soon large vacuoles occur, and the piginent 

 substance is deposited as an irregular layer or in small lumps 



