44 INTRODUCTION. 



of the new cell-wall is gradual and progressive from the old cell-wall 

 inward, instead of being developed simultaneously from a plasma 

 membrane previously formed. Whether in such cases new plasma 

 membranes are formed across the ends of the daughter cells which come 

 in contact with the new transverse cell-wall the author is unable to 

 state. 



THE CENTROSOME AND THE BLEPHAROPLAST. 



As illustrations of karyokinesis in which the spindle arises through the 

 agency of centrospheres I have selected the tetraspore mother-cell of Dic- 

 tyota and the ascus of certain Ascomycetes, because the centrosphere 

 is probably best known in those cells and because the entire develop- 

 ment of the mitotic figure has been followed in great detail. In these 

 plants, as well as in Fucus and certain Sphacelariacece, we have seen 

 that the body which we call a centrosome is one that persists from one 

 cell-generation, or nuclear generation, to another in vegetative and in 

 certain reproductive cells. It seems to be capable of division, and is 

 the centre of radiations that give rise to the karyokinetic spindle. We 

 do not know with absolute certainty that the centrosome divides, 

 although the evidence seems to admit of no other interpretation. 



In addition to the plants just mentioned, centrospheres have been 

 found in some liverworts, in diatoms, and in certain Rhodophycea. 

 In the diatoms, however, the behavior of the centrosome during karyo- 

 kinesis, as described by Lauterborn ('96), differs widely from the 

 typical cases described in the preceding pages. In species of Pinnu- 

 laria, Surirella, and others, Lauterborn finds that the peculiar cen- 

 tral spindle arises from the centrosome by a division or process of 

 budding. " Es scheint mir keinem Zweifel zu unterliegen, dass die 

 Anlage der Centralspindel aus dem Centrosom durch eine Theilung 

 (oder, wenn man lieber will, Knospung) hervorgeht " (1. c., p. 61). 



In the diatoms in question the original centrosome is a relatively 

 large globular body which is the center of a system of beautiful radia- 

 tions. Soon after the budding off of the primordium of the central 

 spindle, the original centrosome, with its radiations, disappears, and 

 what is taken to be the new centrosomes arise near the poles of the 

 spindle and apparently from it. 



So far as the author is aware, such a phenomenon has no parallel 

 among plants, and it is impossible to bring the process of spindle- 

 formation in the diatoms, as described by Lauterborn, into line with 

 anything known in other organisms. 



When we consider the facts alone in the algae and fungi mentioned, 

 we certainly have strong evidence in favor of the doctrine of the genetic 



