B32 The Unity of the Organism 



ture gathered around it. Nothing like an exhaustive review 

 of the observations and hypotheses can be thought of here. 

 But our present interest in it, namely the question of whether 

 or not flagella are originally and fundamentally part of the 

 nuclear system, requires us to acquaint ourselves with some 

 of the main results reached by investigations into the struc- 

 ture, function and origin of the centrosome. 



As regards structure, what most concerns us is whether the 

 minute central granule, deeply stainable in certain special dyes, 

 is the essential thing, and so sliould be regarded as the centro- 

 some, or wliether this granule, togetlier with tlic more volumi- 

 nous, less dense, less easily stainable substance around it, are 

 fundamental so that the whole should be regarded as the cen- 

 trosome. Such an examination of the writings on this question 

 as a general student is able to make almost forces him to con- 

 clude that the application of the names centriole, micro-centrum, 

 cytocentrum, astrosphere, attraction sphere, etc., to the various 

 objects treated under the general designation "cellular cen- 

 ters," is at present largely a matter of personal choice. This 

 results from the great structural variety, taking the whole ani- 

 mal kingdom together, of the parts dealt w^ith, and the meager- 

 ness of positive knowledge as to the functions of these parts. 

 Thus, on the question which cliiefly concerns us now, that of 

 what shall be called centrosome, great difference of view and 

 hence of nomenclatural usage prevails. 



The term centrosome was first used according to Wilson, by 

 Boveri and was applied to a small protoplasmic spherule differen- 

 tiated from the surrounding cytoplasm "in the center of which one 

 or two exceedingly minute spheres, the centrioles, are enclosed." ^ 

 In a word, as originally conceived, the centrosome conformed to 

 the second alternative indicated in what was said above about what 

 a centrosome really is. But later investigations produced facts, 

 chiefly concerning the penetration of the astral rays during in- 

 direct cell division into the less stainable substance around tlie 

 central granule tliat led many investigators to regard tlie granule 

 alone as the centrosome. Tliis is the position held by E. B. Wil- 

 son in the 1899 edition of The Cell and also by O. Hertwig in 

 his Alloemeine Bi()lo<fie. But Hertwig tells us in the fourth 

 edition (1912) of his book ^'' that the arguments ])roduced by 

 Heidenhain in Plasma und Zelle have convinced him that the 



