TWO TYPES OF DOUBLE-CELLED STRUCTURES. 19 



simple and double celled phases in or^^anisms of intermediate evolu- 

 tionary rank, and tinall\' the double-celled phase is shown to ])e an 

 expansion of the fertilized egg, which constitutes an increasingly 

 large part of the life history as organisms mount higher in the scale 

 of evolutionary progress. 



It is- thus eas}^ to understand why the two t^^pes of double-celled 

 structures have ver}' unequal possibilities of organization. Two nuclei 

 are evidently better than one, but their association is too slight, appar- 

 ently, to gain much of the vital stimulus consequent upon the more 

 effective method of conjugation followed b}' the higher plants, where 

 the chromosomes of two fused nuclei lie in juxtaposition in the new 

 nucleus. The higher organisms have not merel}^ double cells, but, 

 what seems to be vasth^ more important, compound nuclei, a more 

 advanced and energized stage of the sexual process, which enables them 

 to maintain for exceedingly long periods of time the power of growth 

 and subdivision." 



The intercalation of the double-celled structure does not chansfe the 

 order of nuclear events in cross-fertilization, but it may be said to 

 change fundamentally their chronological and ph3^sio]ogical relations. 

 The true historical sequence of conjugation is plasmapsis, karyapsis, 

 and synapsis, but the apparent and practical sequence in the higher 

 plants and animals becomes synapsis, plasmapsis, and karyapsis, the 

 synapsis which ends one conjugation being followed closely by the 

 plasmapsis which begins another. The suspension of nuclear changes 

 for vegetative growth no longer occurs between S3^napsis and plas- 

 mapsis, but between karyapsis and synapsis, the double-celled, para- 

 gamic structure being ])uilt, as already stated, on a new and highly 

 sexual plane, that is, out of cells in a state of prolonged sexual union. 



If, as ma}^ be supposed, the benefit of synapsis lies in the making of 

 new associations of the ancestral chromatin elements, it is obvious that 

 the bringing of two such newly energized nuclei together would pro- 

 duce a condition which, for want of other words, might be called a 

 multiple vital tension. The double-celled type of structure involves, 

 therefore, not merel}' a transfer of emphasis to a new part of the life- 

 cycle, but a new and improved sexual process, which raises the bio- 

 logical equation to a higher power. From this standpoint it is ol)vi- 

 ous that the morphological diversities of sex have a fundamentally 

 important and truly ph3'siological function in building up and main- 

 taining the efficiency of the complex organization of the higher plants 

 and animals. It is as illogical to ascribe the internal diversit}' of 



«As noted before, some organisms, such as Caulerpa and Acetabularia, show a con- 

 siderable degree of evolutionary progress, and have not as a matter of fact attained 

 the cellular type of organization at all; they may, however, be found to have double 

 nuclei and to be very striking examples of the expansion of the fertilized egg. 



