242 THE AMERICAN NATURALIST. [VOL. XXXIX. 



life history, which must take place with as much regularity as 

 the normal development of any organ. As a matter of fact, 

 our knowledge of the structure of sexual elements and the events 

 of sexual phenomena is almost wholly morphological and for the 

 present at least it seems safer to treat and define sexuality from 

 a morphological standpoint. 



Under asexual cell unions and nuclear fusions we shall include 

 a number of interesting phenomena which can be arranged in 

 three groups : ( I ) cell fusions which have apparently no sexual 

 relations ; (2) cell fusions which are substitutes for a normal 

 ancestral sexual process now suppressed ; and (3) extraordinary 

 modifications of what may have been originally sexual processes 

 but which at present serve some peculiar and special function. 



In the first group will be included the extensive union of 

 swarm spores, or the amoeboid elements derived from such, 

 best illustrated in the development of plasmodia ; also such cell 

 fusions as are clearly for nutritive purposes, as is the union of 

 the sporophytic portion of the cystocarp of the red algae with 

 auxiliary cells and probably also the fusion of sporidia in the 

 smuts and the conjugation of yeast cells. The second group 

 embraces the interesting fusions of the nuclei in teleutospores 

 of the smuts and rusts and in the basidium with the previous 

 history of the paired (conjugate) nuclei in the mycelium, perhaps 

 also the nuclear fusions in the ascus, and such cell unions as 

 have been reported preliminary to the apogamous development 

 of the fern sporophyte. The third group includes the remarka- 

 ble phenomenon in the embryo sac, the double fusions of the 

 polar nuclei and the triple fusion of these with the second sperm 

 nucleus, frequently called "double fertilization." 



The well known union of the swarm spores of the Myxomy- 

 cetes as amoeboid cells (myxamcebae) to form the plasmodium is 

 one of the best illustrations of a fusion of protoplasm without 

 sexual significance. In this general union of hundreds and per- 

 haps thousands of small cells there are no nuclear fusions so far 

 as is known, but simply the merging of the cytoplasm to form a 

 large multinucleate unit. The whole phenomenon indicates a 

 cooperative process which is probably economical of nutritive 

 functions in the semiterrestrial conditions under which plas- 



