460 THE BEPEODUCTIVE SYSTEM 



The outline on the preceding page summarizes in tabular form the 

 main facts regarding the development of the reproductive system. 



In both sexes the Wolffian duct proximally sprouts a duct which dilates 

 and subdivides distally, meanwhile separating from the parent duct and 

 making a secondary connection with the developing bladder. The subdi- 

 visions elongate to form the collecting portion of uriniferous tubules, and 

 unite with ontogenetically distinct tubules (the secretory portions) to form 

 the complete tubules. The dilatation and earlier subdivisions become the 

 pelvis and calyces, and the proximal portion of the original anlage persists 

 as the ureter. 



GAMETOGENESIS 



The study of the sex or genital glands, the ovary and the testis, 

 is perhaps best approached by way of a consideration of the mechanism 

 by which they perform their respective specific functions, namely the 

 production of ova and spermatozoa ripe for union. The common process 

 is known as g am eto genesis. The end products are the male and female 

 gametes, or sperm and ova. In the male the process is known as 

 spermato genesis, in the female, odgenesis. The act of subsequent union 

 .of the gametes is called fertilization, and the fertilized egg is the zygote. 



The result of gametogenesis is the preparation of a primary germ- 

 cell for union with a gamete from the opposite sex. The essence of 

 the process is known as maturation, and involves prominently mitotic 

 cell division. However, the method of division is not of the simple 

 homeotypic type, where a chromosome simply divides longitudinally into 

 two daughter chromosomes, but is of the type called Jieterotypic, the 

 chief characteristic of which is the formation of tetrads. The latter 

 are of various sorts, all, however, characterized by a four-lobed condition 

 representing a quadripartite double or bivalent chromosome (Plate B, 

 figs. 15-18, page 467). 



Both sperm and egg trace their ancestry back to primordial germ 

 cells, indistinguishable in the sexually undifferentiated organism, ex- 

 cept for a difference in chromosome content, which difference is com- 

 monly indiscernible. In certain instances, e.g., Ascaris, the germ cell 

 can be distinguished from the soma cell at the two-cell stage. In a 

 number of vertebrates, e.g., dogfish, turtle, etc. (Allen), the primordial 

 germ-cell has been traced from a position among the entoderm cells 

 lining the gut of the young embryo through a migration into the dif- 

 ferentiating gonad } ovary or testis. Swift (Amer. Jour. Anat., 15, 4, 



