THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 441 



The Ovary. If the individual is to be a female, specific changes occur in the genital 

 ridge which lead to the formation of the ovary (see page 408 in text). The beginning of these 

 changes can be seen in pig embryos of 25 to 30 mm. (or human embryos of about 25 mm.), 

 prepared according to the technic given under "The Genital Ridge" (see p. 440). The 

 ovary forms a fairly large ridge attached to the medial side of the mesonephros by the meso- 

 varium, but is relatively shorter than the original genital ridge. Cords of epithelial cells 

 (the sex cords) extend into the underlying mesenchymal tissue and are composed of small, 

 dark cells and larger, clearer cells (the primitive ova, sex cells). It should be borne in mind 

 that these cords of cells are not Pfluger's egg cords (which are formed later), but that the 

 ones in the anterior end of the ovary are the forerunners of the rete cords, the ones in the 

 middle region the forerunners of the medullary cords. 



For the study of the formation of Pfluger's egg cords, egg nests and primary Graafian 

 follicles, remove the ovary from a human foetus of seven to eight months, or from any mam- 

 malian embryo at a corresponding stage of development, fix in Flemming's fluid, cut thin 

 sections in celloidin and stain with Heidenhain's haematoxylin. 



For the study of the later development of the Graafian follicles, fix the ovary of an adult 

 cat or dog in Orth's fluid, cut celloidin sections through the entire organ and stain with 

 Weigert's haematoxylin and eosin. These sections are usually satisfactory for the study of 

 corpora lutea. 



The Testicle. The beginning of the changes that differentiate the testicle from the 

 ovary can be studied in sections of male pig embryos of 25 to 30 mm., prepared according to 

 the technic given under "The Genital Ridge." As in the ovary (see above), trabeculae 

 of epithelial cells extending into the underlying mesenchymal tissue constitute the sex cords, 

 but the sex cells (forerunners of the spermatogonia) are less clear and resemble the undiffer- 

 entiated epithelial cells. The sex cords in the anterior end of the testicle are the forerunners 

 of the rete testis (which unites with the mesonephric tubules) and the straight tubules; those 

 in the middle region are the forerunners of the convoluted seminiferous tubules. 



Sections of a testicle at a later period (at birth, for example) are very instructive. Fix 

 the gland in Orth's fluid, cut longitudinal sections of the entire organ through the rete testis, 

 including also the epididymis, and stain with Weigert's haematoxylin and eosin. 



The descent of the ovaries and testicles can be demonstrated in dissections of human em- 

 bryos of three months and more. If the dissections are carefully made, the bladder and 

 urachus, the urogenital sinus, and the various ligaments can be identified. 



References for Further Study. 



ADAMI, J..G.: The Principles of Pathology. Vol. I, 1908. 



AICHEL, O.: Vergleichende Entwickelungsgeschichte und Stammesgeschichte der 

 Nebennieren. Arch. f. mik. Anat.. Bd. LVL, 1900. 



ALLEN, B. M.: The Embryonic Development of the Ovary and Testis in Mammals. 

 Am. Jour, of Anat., Vol. Ill, 1904. 



BEARD, J.: The Germ-cells of Prisciurus. Anat. Anz., Bd. XXI, 1902. 



BEARD, J.: The Morphological Continuity of the Germ Cells in Raja batis. Anat. Anz., 

 Bd. XVIII, 1900. 



BONNET, R.: Lehrbuch der Entwickelungsgeschichte. Berlin, 1907. 



EIGENMANN, C. H.: On the Precocious Segregation of the Sex-cells of Micrometrus 

 aggregatus. Jour 0} Morphol., Vol. V, 1891. 



FELIX, W.: Entwickelungsgeschichte des Excretions-systems. Ergebnisse der Anat. 

 u. Entwick., Bd. XIII, 1903. 



