XI 
ON THE RELATIVE SENSITIVENESS OF FISH EMBRYOS 
IN VARIOUS STAGES OF DEVELOPMENT TO LACK 
OF OXYGEN AND LOSS OF WATER! 
Ir is probable that the series of successive changes in 
form which we call the development of an animal embryo is 
accompanied by a corresponding series of physiological 
changes. While we are well acquainted with the changes in 
form, so far as their mere morphology is concerned, we know 
but little concerning the changes in the physiological reac- 
tions of the embryo in its various stages of development. 
It is a well-known fact that the embryo has a greater vital- 
ity than the completely developed animals.’ Systematic 
investigations, however, are lacking as to whether this vital- 
ity decreases steadily with the progress of the development 
of the embryo, and as to whether this decrease is the same 
toward different variables. In order to obtain an answer to 
these questions, I studied the relative sensitiveness of the 
fish embryo (Fundulus) to lack of oxygen and loss of water 
in different stages of its development. I found in general 
that the embryo is the more sensitive to lack of oxygen, the 
older itis. Yet the sensitiveness increases more rapidly at 
first than later. On the other hand, the experiments on the 
effect of withdrawal of water gave a totally different result. 
The germ of the embryo is much more sensitive to loss of 
water in the first stages of its development (during cleavage 
and before the beginning of the formation of the embryo 
proper) than after the formation of the blastoderm, and 
its sensitiveness decreases with the increase in the devel- 
1 Pfliigers Archiv, Vol. LV (1894), p. 530. 
2Zuntz, Pfliigers Archiv, Vol. XIV; and PFLUGER, ibid. 
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