360 SCIENCE PROGRESS. 



exposure to suitable conditions, either sexuality or vegeta- 

 tive propagation ; an unlimited number of vegetative or 

 sexual generations may be produced in succession, and the 

 operator may at will stop the succession by inducing under 

 suitable conditions the alternative. Here then is no 

 phenomenon of "alternation of generations" in the true 

 (antithetic) sense, since the very essence of this consists in 

 an obligatory succession of certain phases which are not to 

 be modified at will by varied external conditions. 



Such an obligatory succession is, however, found in a 

 rudimentary form in certain other green Algat, as for 

 instance in Sphceroplea, Aidogonium, and Coleochcete. Here it 

 appears invariably the case that the fertilised ovum (zygote) 

 does not germinate directly so as to produce a single new 

 individual, but there is a division of the zygote into four 

 or more parts ; each of these on separation produces by 

 germination a new individual similar to the parent. The 

 constancy of this event would point to its having some deep- 

 seated importance ; but among these plants the theory is 

 in advance of the facts, and we are waiting for the actual 

 observation of details; it would nevertheless seem probable 

 that the explanation of the constancy is to be sought for in 

 the condition of the nuclei after fertilisation. We know in 

 other more highly organised plants that the nucleus of the 

 zygote is the result of fusion of the male and female nuclei, 

 and that on division it shows, as the result, twice the num- 

 ber of chromosomes which appears in the divisions preced- 

 ing fertilisation. Such an increase could not be carried on 

 indefinitely through a series of generations, but a return is 

 necessary in some way to the original nuclear condition be- 

 fore fertilisation. In higher plants this has been found to 

 take place by a process of numerical reduction, and it is 

 believed that this process is intimately connected with those 

 divisions which form a constant factor in the life-cycle of 

 such Alorse as those above named. 



It may be noted meanwhile, in the absence of direct 

 observations on the Confervoidea;, that peculiar divisions, 

 which may well result in a reduction, are seen on the 

 germination of the zygote in certain Conjugatese (see 



