270 



PRINCIPLES OF EMBRYOLOGY 



The reproductive organs of vertebrates develop in rather close associa- 

 tion with the kidneys and their ducts. We shall not deal with them here; 

 recent summaries of the literature may be found in Nelsen (1953) and 

 Nieuwkoop (1946). The germ-cells themselves originate at considerable 

 distances from the glands in which they eventually lie, and reach 

 them after performing a peculiar migration, as isolated mobile cells, 

 through the intervening tissues of the embryo. The place of their origin 

 seems to be surprisingly different in different groups ; probably the lateral 













Figure 12.9 



A shows the epitheUum of the submandibular gland of the mouse growing 

 in tissue culture combined with mesoderm from the same source: the epi- 

 thelium is forming typical tubules. In B the epithelium is lying on top of a 

 porous membrane, below which is mesoderm from another source (maxil- 

 lary region) : no morphogenesis. C is a similar culture but the mesoderm 

 below the membrane is from the submandibular gland: tubule formation 

 has been induced, although the morphogenesis is somewhat abnormal. 

 (After Grobstein 1953.) 



mesoderm in urodeles, the posterior dorsal endoderm in Anura, endoderm 

 lying anterior to the head in birds, yolk-sac endoderm and mesoderm from 

 the posterior end of the primitive streak in mice. An introduction to the 

 literature can be found in Nieuwkoop (1949), Willier (1950) and Chi- 

 quoine (1954). 



9. The limbs 



The limbs first appear as slight external swellings which fairly rapidly 

 elongate. In their early stages they consist of condensations of mesoderm 



