JOHN R. RAPER 31 1 



after it has become applied to the oogonial initial or some other soHd 

 surface. It is quite probable that hormone C is in actuality a complex of 

 two or more distinct substances. 



Once the antheridia have become delimited they secrete yet another 

 substance, hormone D, which causes the oogonium to be delimited 

 by the formation of a transverse septum across the base or stalk of 

 the globular initial, and the reorganization of the protoplasmic contents 

 of the oogonium to form a number of uninucleate, spherical gametes 

 (oospheres or eggs). Again in this stage more than a single hormone may 

 well be involved. 



The entire sexual reaction, except for the transfer of nuclei in fertiliza- 

 tion, progresses in a normal manner between certain compatible partners 

 separated by a permeable membrane. In other matings (with or without 

 an interposed membrane) the reaction stops at earlier phases correspond- 

 ing in each particular case with the specific action of one of the several 

 hormones. Interspecific or even intergeneric matings between hetero- 

 thallic species and various homothallic forms likewise proceed to definite 

 end points in the sexual progression, each characteristic for the specific 

 mating and again to stages corresponding to the specific actions of single 

 hormones. Actually there is considerable evidence that the coordination 

 of the sexual processes in the homothallic forms is similar, excepting 

 hormone specificities, to that of the male plus female mechanism in the 

 heterothallic forms (50). 



Isolation and chemical identification has not as yet been possible 

 for any of the hormones involved in the sexual reaction of Achlya. 

 The physical and chemical characteristics of hormone A have been 

 worked out in considerable detail, and enormous enrichment of the 

 active compound has been achieved: from 1,440 liters of culture fluid 

 of the female plant of ^. bisexualis, 37 per cent of the initial activity was 

 concentrated to 0.0002 grams. This material was active in inducing 

 antheridial hyphae on male plants of A. ambisexualis in a concentration 

 of less than io~^^ (51). The nutritional requirements of and environ- 

 mental factors affecting hormone A production have been intensely 

 studied but much more information is needed (1,48). The chemical and 

 physical properties of hormone A' have been subjected only to pre- 

 liminary investigation. Isolation and/or identification of any of the 

 hormones oi Achlya will depend on the availability of far larger quantities 



