THE ORGANS OF THE MIDDLE GERM-LAYER. 381 



toidin crystals, which now give to the body an orange-red color. The' 

 connective tissue, originally with an abundance of cells, begins tO' 

 shrivel, as in the formation of a scar ; as a result of these various- 

 processes of degeneration the yellow body, which projects beyond the 

 surface of the ovary, begins to become considerably smaller, and is 

 finally converted into a firm connective-tissue callus, which causes- 

 a drawing in at the surface of the organ. 



When fertilisation has not occurred, the same metamorphosis 

 and processes of growth it is true take place, but the false corpus, 

 luteum remains very much smaller. This is probably due to 

 the fact that the afflux of blood to the sexual organs is very much 

 less when there is no fertilisation than in case pregnancy takes 

 place. 



In addition to the tubes of Ppluger, — which arise from the 

 germinal epithelium and produce the primitive ova, — in most classes 

 of Vertebrates epithelial cords of another kind and another origin, 

 enter into the composition of the ovary. As has been observed by 

 various persons in Amphibia, Eeptiles, Birds, and Mammals, there 

 grow out from the Wolfiian body, which lies in the immediate 

 vicinity, epithelial shoots, the " sexual cords of the primitive kidney,"" 

 and these penetrate toward the developing ovary even as early as 

 the beginning of the intergrowth between germinal epithelium and" 

 connective tissue. They arise from the epithelium of the Malpighian 

 corpuscles, as Braun has shown for Reptiles, Hoffmann for Amphibia, 

 and Semon for Birds. In Mammals, in which at present their sub- 

 sequent fate has been most accurately traced out, they then unite 

 with one another into a network at the base of the fundament of 

 the ovary, which protrudes as a ridge into the body-cavity, and, 

 pursuing tortuous courses, grow into contact with the tubes of 

 Pfluger. Whereas in Mammals the cortex of the ovary is de- 

 veloped out of the latter, the former share in the composition of 

 the future medullary substance, and are on that account designated 

 as medullary cords. In the vicinity of the folUcle they remain solid,, 

 whereas the part near the primitive kidney acquires a cavity which 

 is surrounded by cylindrical cells. 



The medullary cords exhibit in different species of Mammals, 

 diflferent degrees of development, as the comparative investigations of 

 Hakz have established. In some animals, e.g., in' the Pig and Sheep,, 

 they reach only to the base of the ovary, and therefore remain sepa-- 

 rated from the tubes of Pfluger by a wide space ; in others they- 

 grow out into the vicinity of the latter, and in part apply themselves- 



