M. WESTERGAARD AND H. HIRSCH 



sex organs, the so-called protoperithecia, which after fertilization develop into 

 perithecia. 



It might a priori be considered possible to select growth as such for our purpose, 

 because the growth type of Neurospora is easily modified by changes of substrate 

 (Tatum, Barratt and Cutter, 1949) and because a number of morphological mutants 

 of widely different growth type are available (Barratt and Garnjobst, 1949; M. and 

 H. K. Mitchell, 1952; and others). The study of the biochemical genetics of some of 

 these mutants is now in progress (Mitchell, Mitchell and Tissieres, 1953) but it is 



Conidia 



Conidia 



Fertilization j>f protoperit/, 



v 



HYPHAL FUSION 



Figure I. Diagram of the life cycle of Neurospora. (From Beadle in 

 Amer. Scientist, 1947.) 



still too early to form an opinion of the possibilities of this system for studying prob- 

 lems of differentiation. As is well known a similar system has been studied intensively 

 in yeast by Ephrussi and his group. The implications of their results on general 

 problems of morphogenesis has recently been summarized by Ephrussi (1953). 



The second differentiation process in Neurospora which might be selected for a study 

 of this kind would be the formation of conidia. This also is strongly influenced by the 

 composition of the substrate, and a number of mutants are available which either 

 form abnormal conidia or fail to form any at all. It is also known that differentiation 

 of conidia is associated with the production of yellow pigments identified as carote- 

 noids (Haxo, 1949; Sheng and Sheng, 1952). However, everybody familiar with 

 work on Neurospora will know that the formation of conidia is a very complicated 

 process involving the production of both macroconidia and uninucleate microconidia, 



172 



