86 LIFE IN NATUEE. 



Every organ of the body begins in this very way : 



by a curved projection of the growing substance. 



Let us look, for instance, 



-Tig. 12. 



at the first-formed organs 



j n f.j ie development of the 



Transverse section of the dorsal 

 plates. The dotted lines repre- chicken Within the egg. 



sent the enclosing membranes 



(after Wagner). Figg> 12 Jmd 13 repr esent 



them in section : they are slight elevations, and are 

 called the " Dorsal Plates," because they are gradu- 

 ally developed into the spinal column. 



These elevations are formed out of a layer of cells 

 called the " germinal membrane," from which all the 



Fig. 13. 



6 



The same, at a more advanced period of development (after Todd and Bowman.) 

 a Dorsal plates. 6 Commencement of a similar fold in another layer of 

 membrane. 



parts of the bird are gradually evolved. It is repre- 

 sented in Fig. 13. Can we help asking whether this 

 may not be a case like that of the growing pea? 

 Whether these n'cfaes are not formed because the 



