115 SEX IN MICROORGANISMS 



cium, in which clumping is Hkewise associated with agglutination of 

 the ciha, have been equally unsuccessful (Metz, 1948). 



In 1927 Schulze reported some type (3) activity in Chi or o- 

 gonium filtrates, possibly attributable to their depleted nutrients. 

 Apart from this uncertain case, sex substances of types (3) (gam- 

 ones*) and (4) (termones) have apparently been reported in unicel- 

 lular algae only by Moewus, whose published results contain a num- 

 ber of discrepant features. They have been reviewed extensively by 

 Thimann (1940), Chodat and de Siebenthal (1941), Murneek (1941), 

 Sonneborn (1941, 1942), Hartmann (1943), Lang (1944), Cook 

 (1945), Smith (1946, 1951b), Raper (1952), and by Moewus himself 

 (1941, 1950a,b). For various reasons it is considered unprofitable to 

 discuss them further here. 



THE COURSE OF THE MATING PROCESS 



Chemotaxis 



Except in such aplanogametic algae as diatoms and desmids, and 

 rare exceptions like the strange Chlaiuydovwims sp. described by 

 Pascher (1918a), fertilization is accomplished in unicellular algae as 

 a result of the motility of flagellated gametes of one or both sexes. 



In oogamous organisms, a common device promoting fertilization 

 is chemotaxis. The immobile eggs secrete soluble substances which 

 diffuse into the surrounding medium, and, by swimming up the dif- 

 fusion gradients, the antherozoids tend to approach the female 

 gametes. The sperm attrahent in Sphaeroplea eggs can be adsorbed 

 on cotton fibers (Pascher, 1931-32), but its chemical nature has re- 

 mained unknown. The discovery that the chemotactic agent in certain 

 fucoids is volatile and may be akin to hexane (Cook and Elvidge, 

 1951) is one of the most remarkable of recent contributions to this 

 field. 



According to Pascher (1931-32), chemotaxis may also be in- 

 volved in the mating process of Chlamy domonas pauper a, which is 

 homothallic and isogamous. In this species gametes which come to rest 

 early act as females, and it is only after their swimming has been 

 arrested that other gametes, acting as males, bes^in to cluster around 



* Moewus (1941) refers to the agents responsible for type (2) reactions 

 as ganiones. 



