14 EVOLUTION OF CELLULAR STRUCTURES. 



postponement of some stage of sexual fusion, for if the final stage 

 is once reached and the chromatin fuses, no further growth is possible, 

 and a new veneration is inaugurated automatically. When .sexual 

 fusion is immediate and complete, i. e., when nuclear fusion follows 

 close on cell conjugation and is in turn at once succeeded by chromatic 

 fusion, no development of the oospore can occur; it simply breaks up 

 into four spores. Such was once the fate of the eggs of all organisms, 

 and such is still their fate in the lower plants. All development of the 

 fertilized egg other than a simple splitting into four spores is due 

 to an arrest of the process of sexual fusion which permits its expansion 

 into a mass of dou])le cells, such as constitute the bodies of higher 

 animals and plants. " It is, however, clear that the effect of such an 

 arrest in the process of sexual conjugation and consequent intercala- 

 tion of a double-celled phase in the life history of the organism is to 

 lengthen the life cycle; it lessens the number of generations instead 

 of making more of them. 



Notwithstanding half a century of endeavor, l)otanists and zoolo- 

 gists have not yet found in the higher animals an}^ definite homologue 

 of the so-called antithetic alternation of generations discovered by 

 Hofmeister^ in the archegoniate plants. The whole idea of alternat- 

 ing generations must, however, be abandoned and emphasis placed 

 instead on the expansion of the oospore or fecundated egg into a 

 double-celled phase that comes to occupy a larger and larger part of 

 the life cycle as organisms mount higher in the scale of evolutionary 

 progress. It then becomes evident that in higher animals (Metazoa)' 

 the expansion of this phase has gone so far that the simple-celled stage 

 has been completely suppressed, and in consequence their life history 

 is as free from alternating phases as that of the lower plants, though 

 for a very different reason. The lower groups show no expansion of 

 the fertilized egg. The higher animals consist of nothing else.*" 



« It is clear that the expansion of the fertiHzed egg could occur in siphonaceous 

 algfe and fungi without any cross walls forming between the nuclei as they arise by 

 sul)division. The mature thallus of an Acetabularia is obviously the enormously 

 expanded syngamete and may or may not contain double nuclei. On the other hand, 

 the Infusoria may be found to consist of one double cell, the successive cell genera- 

 tions not adhering to form a tissue. 



^Hofmeister, W., Vergleichende Untersuchungen der Keimung, Entfaltung und 

 Fruchtbildung hoherer Kryptogamen und der Samenbildung der Coniferen. Leip- 

 zig. 185L 



•^This fact is obscured, but not negated, by the splitting uj) during chromosome 

 reduction of the egg and sperm mother cells of animals into four gametes, which are 

 simple cells, but which are no longer capable of further development unless they 

 conjugate. As previously noted (p. 1.3, footnote (i), these two cell generations occur- 

 ring during chromosome reduction constitute a transition stage between the old and 

 the new generations and can not properly be classed with the simple-celled phase. 



The occurrence of alternating i)hases in the life history of an organism indicates 

 that it is in an unstable evolutionary condition, since it has not yet attained the 



