134 EXTERNAL FACTORS III. 8 



Again, the notochord may be formed from the whole thickness of 

 the archenteric roof (cane-sugar) recalling the mode of its develop- 

 ment in Urodela and Petromyzon (Fig. 69) ; the solid medullary 

 tube observed in potassium chloride and other salts reminds one of 

 the rudiment of the nervous system in Teleostei and others, while 

 the mode of closure of the medullary tube in, for example, some 

 of the magnesium salts resembles that observed in AmpJtioxus ; 

 the formation of notochordal tissue from the wall of the neural 



FIG. 70. Formation of vacuolated notochordal tissue in the medullary 

 tube of the Frog embryo under the influence of urea (1-6%). Underneath 

 the notochord is the subnotochordal rod. 



tube and the roof of the archenteron (Fig. 70) in strong solutions 

 of urea (1-17 % to 1-56 %) shows that the prospective potentialities 

 of these organs are not yet fixed, while the development of an 

 optic cup without a lens in urea, sodium chloride, and sodium 

 bromide demonstrates that the formation of the former is inde- 

 pendent of that of the latter of these two parts of the eye. 



The grey degeneration of the exposed part of the medullary 

 plate (due .to the distribution of the pigment throughout the 

 cell-body), the protrusion of cells (' f ramboisia ' of Roux), and 

 disintegration of the epithelium which is so characteristic in 



