26 



GENERAL CONCEPTIONS. 



from the embryo. The cells then break apart from one another and gradually 

 become separated, and migrate by unknown means, first over the wall of the intes- 

 tine, which has meanwhile been differentiated, then over the surface of the mesen- 

 tery into the anlage of the genital gland. During their entire migration they are 

 lodged in the mesothelium, and when they have reached their final destination they 

 are still in the mesothelium of the genital anlage, where they remain until finally 

 differentiated in the adult. The epithelium, with the germ-cells in their definite 

 position in it, is called the germinal epithelium (compare page 25). The germinal 

 epithelium has been observed in all vertebrates, but the origin of the germ-cells in 



FIG. i. SECTION ACROSS THE POSTERIOR PART OF AN EMBRYO DOG-FISH (SQUALUS ACANTHIAS). TRANSVERSE 



SERIES 463, SECTION 147. 



Ect. Ectoderm. Ent, Entoderm. Md, Medullary tube. Mes, Mesoderm. Nch, Notochord. x, Cellular strand 



connecting the germ-cell cluster with the yolk. 



mammals is entirely unknown. The hypothesis may be accepted, that they arise in a 

 manner essentially similar to that known in the dog-fish. For some of the theories 

 based on the known development of the germ-cells, see page 28. 



The existence of the germinal epithelium has long been known, and its charac- 

 teristics have been described in many text-books. The germ-cells in the germinal 

 epithelium are known also by the names of "sex-cells n and "primitive ova." The 

 transformation of these cells into true ova has been traced in a great many forms, 

 so that the transformation may be considered as demonstrated conclusively for all 

 vertebrate animals. It is further believed that the germ-cells also give rise to the 

 male elements, playing in the formation of the testes a role similar to that which 



