44 



ASCOMYCETES 



[CH. 



that each of them is aheady diploid, the product of a sexual fusion, and 

 capable of independent meiosis. In Helvetia crispa synapsis takes place 

 after the nuclei have fused, but the two spiremes contract in separate masses 

 at opposite ends of the nuclear area (fig. I2)V In other investigated species 

 only one synaptic knot is formed. 





Fig. 12. Helvetia crispa Fr.; fusion nuclei in ascus showing 

 the contraction of tlie chromatin in two separate masses, 

 X 2000 ; after Carruthers. 



The third Division in the Ascus. The meiotic phase is followed by a 

 third division in which a further change in the chromosome number has 



been described. This was first re- 

 corded by Maire, in 1905, in the 

 case of Morchella esculenta, and, 

 with certain variations in detail, 

 in two other Discomycetes. In M. 

 esculenta there are eight chromo- 

 somes in the first telophase, and 

 four in the third, and also in the 

 subsequent nuclear division which 

 takes place in the spore. The next 

 case to be brought forward was 

 Humaria rutilans, in 1907, and 

 two more {Ascobolus furfuraceus 

 and Pyronema confluens) were re- 

 corded by Dangeard in the same 

 year (fig. 13). It was soon after 

 suggested (Fraser, 1908) that the 

 halving of the chromosome num- 

 ber which had been observed in these species constituted a second reduction 

 phase, bearing the same relation to the fusion in the ascus that meiosis bears 

 to the fusion of the sexual nuclei. For this reduction the term brachymeidsis 



^ This arrangement may occasionally be found in Humaria rutilans, and occasionally also the 

 two nuclei of the young ascus proceed as far as the second contraction without undergoing fusion. 



f^' 



■v^ 



b 



Fig. 13. Ascobolus furfuraceus Pers.; a. early ana- 

 phase of the first division in ascus showing 14 of 

 16 daughter chromosomes; b. metaphase of the 

 second division showing four chromosomes ; c. third 

 division showing four ; after Dangeard. 



