Januaet 8, 1909] 



SCIENCE 



65 



sense can be found in the production of two 

 forms of spermatozoa? Still worse is the 

 dilemma presented by the parthenogenesis 

 of the bee or ant. If we here assume that 

 the egg eliminates the female tendency in 

 maturation, fertilization should produce a 

 homozygous male, which is contrary to 

 fact. If, on the other hand, we assume the 

 male tendency to be eliminated, partheno- 

 genesis should produce females, which is 

 also contrary to fact. The only escape 

 from this seems to lie in the assumption 

 that if unfertilized the egg eliminates the 

 female tendency, if fertilized, the male.® 

 But can we regard this as probable ? 



VI. A PROVISIONAL FORMULATION OF THE 



BASIS OF SEX-PRODUCTION IN 



ANIMALS 



I think it must be admitted that until 

 these and various other specific difficulties 

 have been satisfactorily met the Men- 

 deliAn interpretation will fall short of giv- 

 ing an intelligible or adequate explanation. 

 The general evidence in its favor is so 

 strong that we may perhaps hope to see 

 these difficulties cleared away by further 

 study. In the meantime it seems to me 

 that we shall do well to hold as closely as 

 possible to what we actually see of the 

 basis of sex-production in the tracheates. 

 What we see is that males are produced 

 from zygotes that contain but a single X- 

 element, females from those that contain 

 two such elements. It is interesting to see 

 how many of the difficulties of the Men- 

 delian interpretation disappear under the 

 assumption, naive though it may appear, 

 that a single X-element in itself causes or 

 determines the male tendency, while two 

 such elements in association create, or at 

 least set free, the female tendency. As 

 far as the animals are concerned, must of 

 the facts that have been reviewed, in re- 

 spect to both fertilization and partheno- 

 " This suggestion is due to Professor Morgan. 



genesis, fall into line with such a view. 

 Assuming its truth, the facts work out as 

 follows. In ordinary sexual reproduction 

 all the unfertilized eggs should after 

 maturation bear the male tendency because 

 one X-element is left in the egg after re- 

 duction. If capable of parthenogenesis 

 with the reduced or haploid number of 

 chromosomes, such eggs should produce 

 males (as appears to be actually the case 

 in the bees and ants). If fertilized by a 

 spermatozoon that lacks the X-element, the 

 egg still produces a male, for the same 

 reason. If fertilized by a spermatozoon 

 that contains this element, the egg pro- 

 duces a female because of the introduction, 

 not of a dominant "female tendency," but 

 of a second X-element. How this operates 

 to produce a female we can hardly con- 

 jecture; but in order to give point to the 

 conception, let us assume that the X-ele- 

 ment contains factors (enzymes or hor- 

 mones?) that are necessary for the pro- 

 duction of both the male and the female 

 characters ; that these are so adjusted that 

 in the presence of a single X-element the 

 male character dominates, or is set free; 

 and that the association of two such ele- 

 ments leads to a reaction which sets free 

 the female character.^" 



^"Many well-known facts indicate that each 

 gamete may transmit both male and feihale char- 

 acters to the offspring. So far as the eggs are 

 concerned (and also those spermatozoa that con- 

 tain the X-element) I am, therefore, of the opinion, 

 expressed by Correns, Morgan and other writers, 

 that every gamete contains factors capable of 

 producing both the male and female characters, 

 and that this is also true of all the zygotes. In a 

 former discussion I suggested the possibility that 

 the same activity that produces a male might, if 

 reenforced or intensified, produce a female. A 

 somewhat analogous quantitative interpretation of 

 sex, based on the nucleo-plasmic relation, has been 

 put forward by R. Hertwig. Such purely quanti- 

 tative interpretations involve certain difficulties 

 that are avoided by the formulation here sug- 

 gested, which approaches more nearly to a Men- 

 delian interpretation. 



