26 Mutations and Evolution 



disjunction of that pair in meiosis, i.e., AB ? + ABB<? , or (2) that 

 an AAB egg (I.e., having 8 chromosomes) is fertilised by a BB sperm 



resulting from a segregation in meiosis in lamarckiana or (3.) 



A RR 



that the segregation in the megaspore AAABB is ^ , the ABB 

 megaspore surviving and being fertilized by a sperm AB, i.e., the 

 normal type, to give an AABBB individual. 



While it is thus possible for a new extra chromosome to arise 

 in the offspring of lata or lata x lamarckiana, yet the chances are 

 considerably greater that the same one will remain the extra one. 

 The probability of such forms as exilis and exundans depending on 

 the same extra chromosome as lata, depends partly on their 

 frequency in the offspring of lata, which is at present unknown. 



Returning now to the /ata-like forms, de Vries (1909) described 

 a form which he called semilata, and this arose three times 

 independently from lata. One of them when selfed gave 358 

 offspring, of which 3 were nanella, 4 lata, and the remainder semilata. 

 There is some reason ' for believing that this form may perhaps 

 have had 16 chromosomes. On the other hand, the writer 

 described what afterwards proved to be a different form 2 under 

 the name semilata. This stands midway between lata and 

 lamarckiana, has 15 chromosomes and is only known to arise from 

 lata X lamarckiana. When selfed this semilata gives semilata 

 (15 chromosomes), lata (15 chromosomes) and lamarckiana (14 

 chromosomes). 1 The relation between these two 15-chromosome 

 types is therefore a close one, and the differences between them 

 can hardly be due to the presence of a different extra chromosome. 

 It is conceivable, for example, that semilata might arise from a lata 

 egg having 7 chromosomes -j- a lamarckiana male cell with 8, 

 derived through a fresh irregular division. But this explanation 

 would appear to apply better to other 15-chromosome mutants. 

 Semilata appears rather like a somewhat modified lata, and the 

 fact that, as shown elsewhere 3 it exhibits a series of stages running 

 on the one hand towards lamarckiana and on the other towards lata, 

 supports this view. We may therefore suppose it originates from 

 an 8-egg + a 7-sperm. 



Another important fact in this connection is that (E. scintillans, 

 another mutant from Lamarckiana, in now known (Hance 1918) to 



1 See Gates, IWlSa, p. Ill, footnote. 



1 I.e., p. 112. 



1 Gates and Thomas, 1914, p. 532. 



