276 Vertebrate Embryology 



glia cells divide but slowly, and the neurones 

 not at all. 



It should be noted that, according to the 

 so-called law of genetic restriction, " differen- 

 tiation acts as a progressive restriction upon 

 further development. Each successive stage 

 of differentiation puts a narrower limitation up- 

 on the possibility of further advance."^ This 

 may be illustrated by the ectoderm. This 

 layer early shows a division into two secondary 

 layers, the epidermal and the nervous. The 

 epidermal layer never forms nervous structures, 

 and the nervous layer never forms epidermal 

 structures. In the nervous layer are soon de- 

 veloped nerve-cells and neuroglia cells ; the 

 former never develop into the latter, nor the 

 latter into the former. Similar illustrations 

 might be given from the entodermal and me- 

 sodermal layers. 



One of the first changes noticed in the 

 ectoblast, besides the separation into two lay- 

 ers, is the formation of certain thickened areas 

 which have been called plakodes. Examples 

 of these local thickenings are seen in the eye, 

 nose, and ear invaginations that have already 

 been described. From the ectoderm are de- 



' Minot, 



