150 CYTOLOGY CHAP. 



and 0. lamarckiana. Those of 0. gigas are 1-5 to 3-8 times as large as 

 those of 0. lamarckiana, averaging about 2-5 times as large (cells of 

 anther epidermis). Since in general the size of the cell is proportional 

 to the surface of the nucleus, which again is proportional to the volume 

 of chromatin (Boveri, 1904, 1905), we may assume that 0. gigas has 

 twice as much chromatin as 0. lamarckiana, and therefore that its 28 

 chromosomes comprise a double set of those present in the parent form. 

 0. gigas is probably therefore a true tetraploid species. 



The origin of such a form as this can be conceived as being due either 

 to (i) syngamy of two diploid gametes, i.e. gametes in whose formation 

 the meiotic division has been omitted ; or (2) a doubling of the chromo- 

 somes in the zygote nucleus owing to fission of the chromosomes not 

 followed by mitosis. 



It is at present impossible to decide between these alternatives. As 

 we have seen, a doubling of the chromosomes may take place under 

 certain experimental conditions (p. 148), and there appears to be no 

 reason to deny that this might happen in nature under abnormal 

 conditions. If the doubling occurred in the undivided zygote cell, it 

 might result in the whole individual being tetraploid. On the other 

 hand, diploid egg cells are normally formed in obligatory parthenogenesis 

 and may possibly occur, and be capable of fertilization, in sexual repro- 

 duction. Diploid microgametes are, however, not known to occur, though 

 the giant spermatozoa occasionally found in many animals are sometimes 

 supposed to be due to the omission of the meiotic division. 



If diploid male and female gametes do occur, they are certainly very 

 rare, and it must be still more rare that two such gametes should meet. 

 In the enormous majority of cases a diploid gamete must meet a normal 

 haploid gamete. This, if syngamy took place, would result in a triploid 

 zygote. Triploid Oenotheras, with 21 chromosomes, have indeed frequently 

 been described (Lutz, 1912 ; Stomps, 1912). They have been observed 

 as " mutants " both in pure cultures of 0. lamarckiana and from crosses 

 between the various Oenothera mutants. Their characteristics vary to a 

 certain extent according to their origin, but in their general growth they 

 appear to stand mid-way between 0. gigas and the ordinary diploid 

 Oenotheras (Stomps, 1912). 



Stomps estimates that three triploid (" Hero ") plants are found 

 among about 1000 plants in certain Oenothera cultures, and that 0. gigas 

 appears at most in the proportion of i : 10,000, though the total 

 number of 0. gigas mutants known is too small for the percentage to 

 be reliably estimated. So far as the figures go, however, the greater 

 rarity of tetraploid forms compared with the triploid varieties favours 

 the view that they have arisen through syngamy of diploid gametes, 

 rather than from a multiplication of the chromosomes in the zygote. 



