ORIGIN AND MORPHOLOGY OF CHLOROPHYLL CORPUSCLES. 249 



the genus Zygnema in which the pyrenoid is always sur- 

 rounded by a sheath composed of numerous starch-grains; but 

 here the division of the sheath may not run parallel with that 

 of the pyrenoid, the result being that one sheath may enclose 

 two or more pyrenoids. 



A new formation of pyrenoids has been directly observed 

 by Professor Schmitz in the simpler case above mentioned, 

 though absolute proof of a question like this is difficult. And 

 apparently a similar process goes on in the case of those of the 

 more complicated type. In general, however, the increase by 

 division of pyrenoids is by far the most common. 



Returning now to the study of the chromatophore at large, 

 it is found that these bodies also multiply by division, and 

 several varieties of the mode of division are described, the 

 most important being division into two, while sometimes a 

 division into more than two may be obsei'ved. Further, in the 

 first case the division may be into equal or into unequal parts. 

 In those cases where there is a single pyrenoid, the division is 

 often closely connected in point of time with that of the whole 

 chromatophore (fig. 15) ; and the comparison might be drawn 

 between this and the behaviour of the nucleus in ordinary cell 

 division. The same argument, however, may be brought to 

 oppose a view of a definite causal dependence of these pro- 

 cesses one on another, as has been already advanced in the 

 case of cell division, and division of nuclei ; viz. that as the 

 division of polynucleated cells is independent in time of the 

 division of the nuclei, so also is the division of chromatophores 

 with many pyrenoids independent of their division. 



Professor Schmitz states boldly that there is never any 

 other increase in number of chromatophores in the 

 cells except by the process of division. Here he is at 

 issue with the statements of many observers. He draws his 

 conclusions, however, from the careful observation of many 

 forms, during different periods of their life. He compares this 

 character of the chromatophore with that of the nucleus above 

 alluded to. A further point in common between these bodies 

 is that of copulation or fusion ; this has been observed in the 



