SECT. 6] 



OF THE EMBRYO 



bryonic body, and a rise in the water-content of the embryo-plus- 

 yolk system. 



The obvious question now arises, what is the origin of the water 

 which helps to build up the tissues of the frog and the trout embryo. 

 Gray's work on the brown trout, Salmofario, provides a graph showing 

 the relative amount of embryo and yolk in the different stages ; this 

 is given in Fig. 234. The wet weight of the embryo in percentage of 



50r 



oTrout 



60 70 80 90 



Days after hatchinci 



Diagram illusbrabing change in water content of Larvae 

 Fig- 233. 



the wet weight of the system embryo plus yolk is plotted agains 

 the time, and the resulting curve is S-shaped. It contributes to 

 the view that the rising water-content of the whole system (shown in 

 Fig. 235, which is also roughly S-shaped), is due entirely to 

 the increasing preponderance of embryonic tissues, which main- 

 tain a constant proportion of water within themselves. "In other 

 words", as Gray says, "as yolk is converted into embryo, water is 

 added from the external environment and the yolk may be regarded 

 as more or less desiccated nutriment. We can therefore conclude that 

 the concepts of ' passive ' and ' active ' growth (Davenport) have no 



