CHAP. 



146 CYTOLOGY 



instead of radially. Hence the first polar body is not extruded, but 

 all the chromosomes — i.e. two bivalents— remain in the egg. At the 

 second meiotic division all these chromosomes enter the spindle, with 

 the consequence that the egg and the single polar body are each left 

 with two univalents instead- of one. When the egg is fertihzed, the 

 zygote nucleus has therefore three chromosomes, two from the egg 

 and one from the spermatozoon. These three chromosomes appear 

 regularly in all the subsequent mitoses in the germ track. As the polar 

 bodies in Ascaris are conspicuous objects for a considerable time in 

 the early development of the embryo, the fact can be verified that 

 these rare embryos with three chromosomes have been produced 

 in this way, for they have only one polar body (containing two 

 univalents) instead of two (one with one bivalent and the other with 



one univalent). 



Some of the Oenothera mutants, e.g., 0. lata, probably owe their origin 

 to an irregular distribution of chromosomes in meiosis, since they differ 

 from the parent forms in possessing an additional chromosome (2^ = 15 

 instead of 14) . 0. lata has been produced several times among the Oenothera 

 mutants, occurring in the frequency of about .4 per cent (Gates and 

 Thomas, 1914). It presumably owes its origin to the union of a normal 

 gamete with one bearing eight instead of seven chromosomes. In the 

 meiosis of this form the chromosomes are normally separated in groups 

 of seven and eight, though sometimes more irregularly, the anaphase 

 groups consisting of six and nine chromosomes respectively, etc. (Gates 

 and Thomas, 1914). Thus forms arise with still more varied chromosome 

 numbers, when gametes bearing these abnormal numbers of chromosomes 

 participate in syngamy. 



In view of that dependence of somatic characters upon the chromatin 

 elements which is required by our thesis, it thus becomes of great interest 

 to know what are the characteristics of the offspring produced by 0. lata. 

 For causes at present obscure, this plant produces practically no fertile 

 pollen grains. Consequently its behaviour in breeding can only be studied 

 in crosses in which other forms are the male parents. Crossed with 0. 

 lamarckiana, in which 2^ = 14, it gives a mixture of lamarckiana and 

 lata offspring (de Vries, 1910). This of course is in accordance with 

 expectation, since all the microgametes have seven chromosomes, and 

 some of the macrogametes seven and others eight. As we have just 

 seen, however, the fifteen chromosomes of 0. lata do not always separate 

 into seven and eight at meiosis, but sometimes into more unequal groups. 

 Lutz (1912) determined the number of chromosomes for fifty-two offspring 

 resulting from a cross between 0. lata and 0. gigas. In the latter 2w = 28. 

 The numbers of chromosomes (2n) found, together with the numbers of 

 plants exhibiting each number, were as follows : 



