SEXUAL REPRODUCTIONS 12-20 



down of biological chemicals in metabolism. This has tended to 

 divert the activity of Neurospora workers to biochemistry rather 

 than on other phases of biology, and has brought their work into 

 the orbit of the biochemist. Yeast affords the best opportunity to 

 unite biochemistry and biology because the morphological compon- 

 ents of the cell have been rather completely described, and it is 

 possible to study very precisely the effect of various nutrients on 

 the cell organelles. 



This is a marked advantage of yeast over Neurospora for bio- 

 logical study; the work which we have been doing in relating the 

 morphology of the cell structure to the metabolic activity is not 

 possible in Neurospora. We have clearly defined the chromosomes, 

 the mitochondria, the nucleolus, and the heterochromatin of the 

 yeast cell, which differs from the conventional cell only in the fact 

 that the heterochromatin is located in a separate compartment. 



The handicap of small size is more than overcome by the fact 

 that the cells can be brought from rest to full division in less than 

 two hours, thus affording an opportimity to observe directly the 

 change in the organelles involved in the initiation of cell division. 

 Furthermore, because they are free cells and can be grown in a 

 variety of media under different environmental conditions, it is 

 possible to alter their fat or protein content or their stored re- 

 serves, thus making possible observation under a variety of cell 

 conditions controlled by the operator; cells organized into tissues 

 or even in a thallus are much more sensitive to alterations of the 

 environment. 



THE NECESSITY FOR DIRECT TETRAD ANALYSIS 



One advantage of yeasts shared by other fungi is the opportu- 

 nity which they afford for tetrad analysis. After the reduction 

 division occurs in the ascus, each of the four haploid nuclei arising 

 after reduction produces a haploid culture which can be grown 

 separately and observed directly for characteristic hereditary 

 differences. This is not possible either with spermatophytes or 

 metazoa. Mendelism is based on the assumption that each sperm 

 mother cell of a heterozygous (Aa) male produces 2 A and 2 a 

 sperm and that each Aa egg mother cell undergoes reduction to 

 produce four egg nuclei (2 A and 2a), three of which pass at random 

 into the polar body and one of which functions as the egg nucleus 

 fusing with the sperm nucleus to produce the next diploid genera- 

 tion. The only direct evidence supporting this assumption is that 

 obtained by tetrad analysis of heterozygous fungal hybrids. Actu- 

 ally, however, the four haploid cultures originating from a single 

 individual diploid cell following reduction do not always produce 

 regular Mendelian ratios. If AAAa tetrads were produced occasion- 

 ally by a fly, they would be difficult or impossible to detect, or to dif- 



