DEVELOPMENT OF REPRODUCTIVE SYSTEM 795 



These seminiferous tubules develop lumina and unite directly with the rete 

 tubules which arise, in the meantime, from cells of the mesonephric strands. 

 The distal ends of the rete tubules join with the Malpighian corpuscles of 

 certain mesonephric tubules. The mesonephric tubules thus united to the rete 

 tubules are, of course, joined to the mesonephric duct. In consequence, these 

 mesonephric tubules become the efferent ductules or vasa efferentia of the 

 testis (Witschi, '21, '29). 



3. Development of the Ovary 

 a. Mammal 



1) Formation of Primary Cortex and Medulla. The early phases of differ- 

 entiation of the ovary varies in different mammalian species. Two features, 

 however, are constant — features that serve to distinguish the differentiating 

 ovary from the testis. One of these features consists of the fact that the ovary 

 is more retarded in its development than the testis; the testicular features 

 appear sooner in the male embryo than do ovarian features in the female 

 embryo. This is a negative difference, but nevertheless, it serves to distinguish 

 the two sexes. Another constant and positive feature, however, is that the 

 germinal epithelium in the ovary retains its proliferative activity, while, in 

 the differentiating testis, this activity is lost in the early stages of differentiation. 



In the cat and rabbit (de Winiwarter, '00, '09), and in the calf and opossum, 

 the first stage of ovarian differentiation is indicated by a second proliferation 

 of sex cords (Pfliiger's cords) from the germinal epithelium (fig. 350H and I). 

 The earlier sex or medullary cords thus are pushed inward toward the hilus 

 of the ovary, and a definite compact primary cortex is established, containing 

 cords of epithelial and germ cells. The medullary cords become broken up 

 in the meantime and are pressed inward in the direction of the forming primary 

 medulla of the ovary. Some of the germ cells of the medullary cords undergo 

 the earlier stages of meiosis but soon degenerate. 



Synchronized with the foregoing changes in the peripheral area of the ovary 

 are transformations within the hilar region, that is, the area of the ovary 

 nearest to the mesonephric kidney. A conspicuous feature of these changes 

 is the ingrowth of mesenchyme and differentiating connective tissue from 

 the mesonephric kidney. Three morphogenetic phenomena accompany this 

 ingrowth: 



( 1 ) Blood vessels grow into the ovary from the mesonephric kidney to 

 form a primitive vascular plexus within the developing medulla. 



(2) A concentration of mesenchymal cells appears in the area between 

 the developing ovary and the mesonephric kidney. This concentration 

 of mesenchyme is the rete blastema, or the rudiment of the rete ovarii. 



(3 ) From the region of the rete blastema radiating columns of mesenchyme 

 and differentiating connective tissue fibers extend outward through 



