286 THE THEORY OF THE GENE 



ence of germ-cells). The development of this organ, after 

 castration, can, I think, be provisionally at least accounted 

 for by a secondary growth and enlargement of the funda- 

 ments of the male organs that are known to be present in 

 the embryonic stage. The maintenance of a testis, even a 

 functional one, in a female body is not in itself surpris- 

 ing, since it is known that pieces of testis, grafted into 

 the body of a female, may continue to develop, and even 

 to produce sperm. 



In general, it appears that the genetic composition of 

 the female bird (present both in the body-cells and in the 

 young ovary) creates a favorable situation for the de- 

 velopment of the ovary, rather than a testis. Conversely, 

 in the male the genetic composition is favorable for the 

 development of the testis. In the male, however, the early 

 removal of the testis does not suffice to call forth the 

 development of structures peculiar to the ovary. 



The Sex of Parabiotic Salamander Twins. 



The union of young salamanders by side-to-side fusion 

 has been brought about by several embryologists. The 

 young embryos taken from the Qgg, just after closure of 

 the medullary folds, have portions of one side of each 

 removed and are then brought in contact by the exposed 

 surface. Their union quickly follows. Burns has studied 

 the sex of the united (parabiotic) twins. He found that 

 members of a pair were always of the same sex ; 44 pairs 

 were both males, 36 pairs were both females. Random 

 union would give 1 pair of males to 2 pairs of male- 

 females to 1 pair of females. Since no double-sexed pairs 

 appear, it follows either that pairs of opposite sexes die, 

 or that the sex of one individual changes over that of the 

 other and, since both male and female pairs were found, 

 the influence is sometimes one way, sometimes the other 

 way. Unless some explanation can be found for such a 



