RECENT ADVANCES IN VEGETABLE CYTOLOGY. 23 



What, we may ask, is the essential structure of the 

 protoplasm, of the nucleus, and of those marvellous bodies, 

 the chromosomes, which reappear at every nuclear division ? 

 What is it that initiates the division of a cell or of its 

 nucleus, and why do some cells go through such complex 

 evolutions whilst others seem to adopt a relatively simple 

 course? What is it that determines that the descendants of 

 one cell shall develop differently from those of another, so 

 as to give rise to this or that tissue system ? Or again, 

 how is the unicellular condition of an infusorian compatible 

 with an intricate and often highly differentiated organisa- 

 tion ? 



These and a host of other questions rise and confront 

 us on the very threshold of our inquiry, and the hints which 

 Nature has dropped for our guidance are at best only 

 obscure ones ; thus the position of the biological investigator 

 contrasts unfavourably with that of the chemist or physicist, 

 inasmuch as he is generally debarred, owing to the very 

 conditions of the bodies he is dealing with, from having 

 recourse to direct experiment ; Nature conducts the experi- 

 ments and he has to remain content with watching- the 

 result, analysing the factors and reconstructing the process 

 as best he can. Nevertheless there is, clearly, no funda- 

 mental distinction between the (so-called) observational 

 and experimental sciences. 



It is, then, only by patient accumulation and careful 

 comparison of all the facts that even a proximate solution 

 of the difficulties before us can ever be reached. Much 

 has been done in collecting the data, and a good deal is 

 known both as to the structure of the cell and the phases 

 through which it passes during its existence. And fortu- 

 nately one generalisation is gradually emerging with in- 

 creasing clearness from beneath the ever-growing pile of 

 detail, and it promises to prove a guide of no small value, 

 namely, that in those processes which we have reason to 

 regard as fundamentally important there exists a surprising 

 degree of similarity between the structural elements of 

 animals on the one hand and of plants on the other. And 

 these points of similarity are now known to be so numerous 



