270 Transactions of the Society. 



exclude both mitochondria and chondrioplasts (Grolgi rods) from the 

 exercise of a direct part in hereditary transmission. The unequal 

 distribution of these elements during cytokinesis and their incon- 

 stant numerical relations (cf. Wilson on Opistkacanthv.s) -, their 

 elimination either wholly or partly from the male gamete either 

 during spermateleosis (Oatenby and Montgomery), or before 

 fertilization (Lillie; ; and finally, their failure to provide any 

 evidence of segregation in gametogenesis (cf. especially Gatenby on 

 Limax), demonstrate (1) that the complex of one-cell generation is 

 not integrated in such a way as to be individually representative of 

 that of another; (2) that in many cases at least the mitochondrial 

 or chondrioplast organization of the zygote has no continuity with 

 that of the male parent. The relation of the maternal and paternal 

 cytoplasmic inclusions provides no means of effecting alternating 

 inheritance. 



Thus to-day the importance of the nucleus in inheritance has 

 been emphasized rather than diminished by the attention which 

 improved technique has directed to other parts of the cell. The 

 immediate problem of genetic cytology therefore concerns the 

 manner in which the nucleus itself functions in the process ; and 

 naturally, the theory of synapsis, or the conjugation of chromo- 

 somes derived from alternate parents preparatory to this segrega- 

 tion in the maturation divisions, occupies a position of central 

 importance in the discussion. Considerations in favour of the 

 recognition of chromosomes as units in the hereditary process have 

 been drawn from cytological studies in relation to sex determina- 

 tion, mutation, and generic hybridization ; and since these have 

 been admirably epitomized by Gates and Doncaster (Q.J. M.S., 

 1914), they do not call for comment in this place. The most 

 fruitful basis however for a discussion of the chromosome hypothesis 

 concerns, whether, in the behaviour of the chromosomes in the 

 germ cycle, we are actually witnessing the mechanism of alternat- 

 ing heredity in operation. This problem has become an increasingly 

 technical one, the issues of which have not been sufficiently 

 criticized. 



In treating of the growth of the modern doctrine of Synapsis, it 

 is convenient to accept as a starting point Sutton's observations 

 (1902) on the chromosome groups of Brachystola, published shortly 

 after the rediscovery of Mendel's laws by Correns, Tschermak and 

 De Vries ; for it cannot be too insistently stated that the whole 

 theory rests primarily upon the fact of chromosomal heteromorphism 

 rather than the behaviour of the nucleus in the meiotic phase. 

 To illustrate the bearing of heteromorphism among chromosomes 

 on the theory of gametic segregation a more convenient example 

 than Brachystola itself is furnislied by Nakahara's recent work on 

 the Stonefly, Perla (1919). The spermatogonia! complex liere 

 consists of ten chromosomes, which may be arranged according to 



