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Proceedings of the Royal Society of Edinburgh. [Sess. 
(testes and phallus) and non-potent female ones, viz. Mullerian hydatid 
attached to the testis, and prostatic utricle, the analogue of the hymen, 
with a varying amount of the rest of the female Mullerian tract. When the 
twinning of a male zygote in black cattle takes place, this potent and non- 
potent complex of the genital organs may be divided so that the potent 
part goes to the potent bull-calf, the non-potent to the free-martin. This 
being the general scheme of the division, we must now account as exactly 
as possible for the various parts in the genital tract of the free-martin in 
Spiegelberg’s case, and these may be grouped as follows for explanation : — 
(a) Testes ; (b) Urinogenital sinus and external genitals ; (c) Rudimentary 
vagina; ( d ) Vesicuhe seminales, vasa deferentia, and Wolffian bodies. 
(a) Testes. — The male zygote gave, of course, sperm-germ cells in the 
sperm-epithelium of its Wolffian ridges, and the future testes became divided 
between the twins. This division took place most probably after the 
primitive sperm-cell mass had formed. The testes are undescended. 
(b) Urinogenital sinus and external genitals. — The former is probably 
derived from the somatic division of the twins. The labia and glans are 
non-potent elements. 
(c) Rudimentary vagina and uterus. — These are undoubtedly derived 
from the Mullerian hydatid and prostatic utricle (PI. II. fig. 4). 
(d) Vesiculce seminales , vasa deferentia, and Wolffian bodies. The first 
two arise from the Wolffian ducts which are present and derived from the 
non-potent elements; and the Wolffian bodies are given in somatic division. 
Thus the free-martin with a potent bull twin is the result of a division 
of a male zygote, so that the somatic determinants are equally divided, the 
genital determinants unequally divided, the potent going to the one twin, 
the potent bull, the non-potent genital determinants to the free-martin. 
The potent organs are dominant, the non-potent recessive, and the Mendelian 
scheme may be figured as follows (diag. 1) : — 
Diag. 1. Male sex gamete X Female non-sex gamete P 
II 
Male zygote F 1 
which if it twins 
may give 
Bull with equivalent soma Bull with equivalent soma and non- 
and potent genital organs. potent genital organs (female type). 
In three of Numan’s cases the sterile twin had testes and a rudimentary 
preputial sheath. This is a rare free-martin where the Mullerian elements 
remain in the potent bull, but the preputial element of the external genitals 
