398 BREAKDOWN OF GENETIC SYSTEMS 



most of which presumably have only one chromosome. Fragments 

 are occasionally found. The pollen is entirely sterile, but nearly 

 10 per cent, of the embryo-sacs are viable and haploid, owing to the 

 failure to form separate daughter-nuclei after division or to less 

 abnormality. 



It will be observed that the first of these divisions has the property 

 of beginning precociously, which in itself enables one to predict its 

 later course on the precocity theory. This division corresponds in 

 certain essentials to the first meiotic division in a haploid. The 

 occurrence of pairing is analogous to that at meiosis in haploid 

 (Enothera [v. Ch. VIII). It does not occur in pollen grains with 

 reduplicated segments or whole chromosomes (Beadle, 1933). This 

 is not surprising, since unbalance as well as particular genes found 

 in inbred stocks will reduce chiasma-frequency in Zea (Randolph, 

 1928; McClintock, 1929 a; D., 1934; Beadle, 1933 a). The 

 appearance of fragmentsis expe cted on the chiasmatype hypothesis, 

 because crossing-over at chiasmata between relatively translocated 

 segments will give new chromosome types large and small. 



The abnormality differs from meiosis in that it is reiterative, so 

 far as the spindle is concerned, while, on the other hand, the centro- 

 meres may never divide at all ; the conditions that determine it persist 

 as long as there are chromosomes to divide, while in meiosis it is 

 assumed that the special conditions determining precocity act once 

 for all at the prophase of the first division and determine the second 

 division only through their effect on chromosome behaviour at the 

 first. 



Supernumerary divisions which are striking in their similarity 

 to those in Zea have been found in a Drosophila pseudo-obscura 

 race-hybrid with a genotypically abnormal meiosis (Dobzhansky, 

 1934). Here a new generation corresponding to the plant gameto- 

 phyte is introduced into the animal's life-cycle after meiosis ; this 

 is associated with meiosis being pushed back two or three divisions 

 in development. Unlike the polymitosis in Zea, the divisions are 

 not precocious and are separated from one another by resting 

 stages during which the chromosomes divide. Moreover, no 

 separation of the daughter cells occurs. Consequently neighbouring 

 mitoses fuse and multipolar spindles are formed. The poles are 



