SEX. 27 



they play in the ovary. The proof that the germ-cells are the exclusive parents of 

 the spermatozoa is difficult to obtain, but most embryologists regard the existing 

 proof as sufficient. 



When a germ-cell is transformed into an ovum, it undergoes great enlargement,. 

 its nucleus is modified, the protoplasm is changed in appearance and becomes loaded 

 with yolk granules, and over the surface of the cell appear two membranes, an 

 inner very thin one, called the vitelline membrane, and an outer much thicker 

 one, known as the zona pellucida. (For a fuller description see page 34.) We 

 thus learn that the germ-cells preserve their resemblance to segmentation spheres 

 only during embryonic life. When they become ova, they pass through a series 

 of important changes in their organization. Since germ-cells also give rise to the 

 male elements, we must say further that in order to produce those elements the 

 germ-cells pass through another series of profound changes. 



It is further known that in order to evolve the sexual elements, both male 

 and female, the cell which is to produce them divides twice, and in a special man- 

 ner, which we designate by the term "reduction division." This process is de- 

 scribed in all the recent text-books of cytology and histology. It does not fall 

 within the scope of this work, which deals with embryology in the strict sense only. 



Sex. 



The sex of an individual depends primarily upon the nature of the sexual 

 glands. The same two, right and left, parts produce the sexual glands in all verte- 

 brates. Each part originally is a limited area of the surface of the cephalad end 

 of the urogenital ridge (compare p. 6) and becomes either a testis or an ovary, 

 and, since the two sides develop alike, the individual is wholly male or female as 

 the case may be. 



As an exceedingly rare anomaly, lateral hermaphroditism has been recorded. In 

 this anomaly there is a testis upon one side, an ovary on the other. This is the 

 only form of true hermaphroditism known to occur in the amniota. 



Each sex is further distinguished by secondary sexual characteristics, in part such 

 as are immediately concerned with reproduction, like the uterus, mammary glands, 

 vas deferens, etc., in part such as are less directly connected with reproduction, 

 such as size, distribution of hair, etc. In the course of development the sexual 

 glands are clearly differentiated before the secondary sexual characteristics appear. 

 Hence arises the question, have the glands a causal relation to the secondary 

 characteristics ? 



The hormone theory is the only one, available at present, to explain such a 

 causal relationship. It is known that various glands produce a so-called internal 

 secretion, which is distributed through the body probably by the medium of the 

 blood and acts upon structures quite remote from the organ producing the secre- 

 tion. Similar chemical products arise also from organs, which cannot be regarded 

 as glands in the usual sense. All of these secretions or products have received the 



