text from the image:

The girl-boy with diffused minor abnormality in physical structure, consisting in approach to the feminine type, is rather a female who has, along with some other male structures, developed testicles and penis in place of the usual ovaries and cunnus. Here it is not so much a case of a female brain in a male body, but of the female brain in a female body with various abnormal developments along the line of male structure. A girl-boy is sometimes even physically perhaps more a female than a male, although the primary sexual determinants and some of the secondary sexual characters are those of the male sex.

[Sex Psychical Rather Than Physical.] In a manner similar to that described by Kurella, the author believes the invert is a transitional form between the complete male or the complete female and the sexually undifferentiated homo seen in the early fœtus. Practically it is all right, but medico-legally it is wrong, to make the genitals the universal criterion in the determination of sex. Medico-legally, sex should be determined by the psychical constitution rather than by the physical form.

The idea of brain sex that he puts forward in his book is different from the current version. Today the argument seems to be that the brain structure is different depending on gender and he believes that the very cells themselves are different for males and females.

—whether any individual shall be a male or a female depends on the result of a battle in the embryo between the female corpuscles or germs of the egg and the male of the spermatozoa. From some cause, perhaps the relative state of vitality of the secretory sexual glands at the time of the formation of the particular egg and spermatozoon, either the female germs or the male germs happen to be the more vigorous, and determine the sex of the unborn. If the fœtus develops into a female, it is because the female germs have devoured the male. For some reason, in exceptional cases, the more vigorous set of cells have not succeeded in devouring the other set entirely, and both kinds coexist in different parts of the same individual throughout his existence.

from page 18