CKLLULAR DEGENERATION AND FORMATION OF PIGMENT. 131 



granules are formed. The young yolk-cell^ instead of pro- 

 ducing normal yolk-masses, becomes coarsely heterogeneous in 

 texture and gradually assumes a dark brown or black colora- 

 tion. The nucleus swells and becomes indistinguishable, and 

 the whole mass (measuring 15 ^ x 11 ju) may subsequently 

 become compact and homogeneous. A fully-formed yolk-cell 

 with radial masses of yolk may sometimes degenerate in a 

 similar manner. The degenerated cells may fuse together in 

 twos or threes, and the whole colony becomes intensely black, 

 and, doubtless, perishes (see text-fig. 10 where the planula and 

 coenosarc are moribund). The yolk-masses have a weak depo- 

 larising power, but on browning and fusing together, to form a 

 more or less homogeneous body, this power is greatly increased. 



The yolk-cells are surrounded by many interstitial cells, 

 but it is not easy to decide whether hypertrophy of this tissue, 

 by an increase in the number of the cells, really accompanies 

 the pigmentation of the yolk-cells. It will be remembered that 

 in Sertularia operculata such hypertrophy appears to 

 occur, but in that species the ordinary interstitial cells them- 

 selves become pigmented. 



From this comparative review of the various examples of 

 hydroid pigmentation we learn that there is a specific reaction 

 of the tissues to certain unfavourable stimuli, and these 

 reactions diifer in the various species. It may be noted that 

 marked differences occur among allied species of the same 

 genus (cf. Sertularia operculata, loculosa, linealis). 

 Thus, as known from experiments with immunity, physiological 

 differences do not necessarily run exactly parallel to morpho- 

 logical differences, and there has arisen the conception of 

 physiological species within a single morphological species. 

 The same principle is seen in the well-known fact that two 

 species of a genus, which morphologically are widely separate 

 from each other, may be capable of crossing ; while, on the 

 other hand, two species which may be very closely similar to 

 one another from a morphological standpoint, may be quite 

 unable to cross. 



The characteristic aspect of the degenerated areas must be 



