UNION OF FREE-LIVING CELLS 293 



similarities between the reactions noted in Protozoa and in higher organisms, 

 but there exist also marked differences. The characteristic feature of organis- 

 mal differentials that they are the same in the various tissues of the same 

 organism and are different in the analogous tissues of different individuals 

 does not apply to unicellular organisms. Furthermore, it is very probable 

 that many genes enter into the constitution of the individuality differentials 

 and of the organismal differentials. In Paramaecia, on the other hand, there 

 are strong indications that the difference in agglutination reactions depends 

 upon single or a few selected genes. It seems then that the reactions between 

 different mating groups of Paramaecia are analogous to the fertilization re- 

 actions in higher organisms and this is also implied in the term "mating re- 

 actions" given to this condition, or they may be compared to the agglutination 

 reactions between different blood cells belonging to different blood groups in 

 higher organisms. 



Reactions similar to those studied in protozoa have also been observed in 

 algae and myxomycetae. In the phycomycetous fungus Achlya, the sexual 

 reaction between male and female mycelia seems to depend upon the action 

 of hormone-like contact or distance substances. Such a substance given off 

 by the female vegetative hyphae induces in the male the formation of antherid- 

 ial branches and the oogonial initials attract the antheridial branches, causing 

 the delimitation of the antheridia. The antheridial branches on their part act 

 on the female vegetative hyphae and here induce the formation of oogonial 

 initials, and furthermore, the antheridia cause the delimitation of the oogonia 

 through the formation of a basal wall. These reactions take a normal course 

 if the male and female organisms belong to the same species, but if male and 

 female belong to different species of Achlya, the reaction sets in but remains 

 imperfect. It stops either at the time of the differentiation of the antheridia, 

 or the female fails to produce oogonial initials in response to the substances 

 produced by the numerous antheridial branches. This indicates a specific 

 adaptation between these distance substances, which transmit the stimuli from 

 male to female, or vice versa, and the mycelial substratum on which these 

 substances act. If the latter and the substratum are derived from different, 

 though related, species, the reaction will be incomplete. 



We may refer here, also, to the very interesting recent investigations of 

 Moevus concerning the motility, chemotaxis and copulation of the gametes 

 of certain green algae. There exist a number of races or species of Chlamy- 

 domonas which show inheritable differences in the mode of reactions of their 

 gametes and the conditions which determine these hereditary differences are 

 localized in the chromosomes of the various races. As a result of these genetic 

 differences race specific substances are produced, which direct the motile 

 gametes in the dark. The extract from individuals of each race or species 

 acts most efficiently on the gametes of their own race or species, and more 

 weakly on the gametes of other races. These specific substances are caroti- 

 noids ; the filtrates contain transcrocetin sugar esters which are responsible 

 for these effects. Also, the sugars which combine with crocetin seem to be 

 specific in the different races. 



