. The development of the chick; an introduction to embryology . lu^ embryos of 1)irds aie not welladapted for the solution of this puzzling question; but in somereptiles and selachia and other vertebrates primitive ova havel^een traced from a \ery early stage of the embryo thi-ough variousmigrations to the germinal ei)ithelium. On com{)arative andtheoi-etical grounds, the view of the independent origin of thej)rimitive ova is pi-eferal)le; but the origin of some at least fromthe peritoneal epithelium cannot be disproved for the chick. Two other constituents enter into the composition of the TH
. The development of the chick; an introduction to embryology . lu^ embryos of 1)irds aie not welladapted for the solution of this puzzling question; but in somereptiles and selachia and other vertebrates primitive ova havel^een traced from a \ery early stage of the embryo thi-ough variousmigrations to the germinal ei)ithelium. On com{)arative andtheoi-etical grounds, the view of the independent origin of thej)rimitive ova is pi-eferal)le; but the origin of some at least fromthe peritoneal epithelium cannot be disproved for the chick. Two other constituents enter into the composition of the THE URINOGENITAL SYSTEM 393 indifferent gonad, the stroma cells and the sexnal cords(segmental or genital cords). The stroma is formed from mesen-chyme situated internal to the germinal epithelium. It is avery nariow layer at first, and is formed, in part at least, by pro-liferation of the germinal epithelium itself, in the same manneras mesenchyme is formed elsewhere by proliferation from themesoderm. The stroma of the gonad is separated from the tubules. Fig. LLt).—Cross-section throiifih tlie <ieiiital ])riinor(liuin of Liinosa sego-cephala. (After Hoffmann, from Felix and Biihler.) The stage is similar to that of a chick embryo of 4i days. fierm., Germinal epithelimn. Mst., Mesentery. S. C, Sexual Posterior cardinal vein. W. D., Wolffian duct. of the Wolffian body by the numerous blood-vessels on the me-dian aspect of the latter. Ip to the middle of the jXMiod ofincubation, and a little later, it is extremely sparse; it increasessubsequently as a result of ingrowth of the blood-vessels andaccompanying coiuiective tissue. 394 THE 1)kv1:lop!ment of the chick The sexual cords ai)|)ear within the fioiiad on tlie fifth (hiy;they are sohd cords of epitliehal cells that fill up the interiorof the gonad and cause it to protrude from the surface of theWolffian body (Fig. 22(5); the cords extend from the germinalepithelium, with which tliey nuiy be in contact, towa
Size: 1656px × 1508px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, bookiddevelopm, booksubjectbirds
