12 EVOLUTION OF CELLULAR STRUCTURES. 



reduction ))ecause c3'tology was approached from the standpoint of the 

 somatic tissues of the higher plants and animals. This current inter- 

 pretation reverses, however, the historical course of events. The 

 reducing division was not an expedient incidental to the adoption of a 

 process of sexual reproduction by organisms previously sexless. It 

 VMS not the redaction to fewer chromosomes^ hut the retention of the 

 douhle numher, that constituted the important step in sexual rejyroducti on 

 and made j^ossihle the evolution of comjilex hiijlter organisms. It is, 

 therefore, not the reducing division, hut the doii})ling conjugation, 

 which should constitute the datum point or base line for tracing cyto- 

 logical homologies. 



THE ELIMINATION OF THE SIMPLE-CELLED PHASE. 



Chromosome reduction brought about b}" synapsis, or the fusion of 

 the chromatin elements, followed by two special nuclear divisions, is 

 not, historically speaking, the beginning- of the sexual process, but 

 the end of it. Chromosome reduction stands in no special causal rela- 

 tion to the subsequent conjugation. The number of cell generations 

 formed between synapsis and conjugation differs greatly in the various 

 natural groups, and merely shows how far the organism still adheres 

 to its old simple-celled life history. Fecundation and synapsis, as the 

 beginning and the end of the sexual process, would seem to ))e directly 

 comparable in all organisms which have developed a double-celled 

 sexual phase. 



From the physiological standpoint, it may be an advantage to dis- 

 pense with the simple-celled phase and thus shorten the period between 

 the chromosome reduction which marks the end of one conjugation 

 and the cell fusion which begins another. S3niapsis relieves organic 

 fatigue bv means of new nuclear configurations, and has been thought 

 of as a stimulant of vital activity" or eiKU'gy of growth, the benetit of 

 which can be secured for the new double-celled structure by ver}' 

 prompt conjugation, as occurs in all the higher plants and animals. 

 This consideration would help to explain the organic inferiority of such 

 a group as the ferns, which, although they have developed a double- 

 celled phase, continue to waste the energy derived from S3'napsis on a 

 worse than useless simple-celled structui'c. 



In all animals above protozoa this rechiction of the simple-celled 

 phase has gone so far as to result in its complete elimination, for the 

 two peculiar nuclear divisions which occur in rapid succession innne- 

 diately after s3'napsis constitute an essential part of chromatin reduc- 

 tion. That these phenomena noted are indissociablv comiected stages 

 in the process of chromosome reduction has been emphasized recentl3' 

 b3^ Farmer and Moore," who propose the convenient term maiosis to 



« Farmer, J. B., and Moore, J. E. S. On the Maiotic Pha.se (Reduction' Divif^ions) 

 in Animals and Plants, in Quarterly Journal ol Microscoi^ical Science, n. 8., No. 192 

 (vol. 48, No. 4), Feb., 1905, pp. 489-5,57, 7->l. SJ ' 



