370 



ON INCREASE IN SIZE 



[PT, m 



Amphibia. 



Frog. In the case of amphibia, where the cleavage in the egg is 

 more or less inclusive of the yolk-laden portion, it is not possible to 

 obtain data for the weight of the embryo itself, for, before hatching, 

 although the protoplasm is constantly increasing at the expense 

 of the yolk, the two elements cannot be separated, and therefore 

 cannot be weighed in isolation. This appears in the figures of Faure- 

 Fremiet & Dragoiu ; Schaper ; Davenport ; and Bialascewicz, and 

 must always be taken into account when differences between species 

 in water-content and other constants are under consideration, for 

 much confusion may be caused by not distinguishing carefully 

 between yolk plus embryo and embryo alone. 



Reptilia. 



Snake. Bohr's very few figures on Coluber natrix are all that are avail- 

 able. (Appendix i. Table 2.) 



Birds. 



Chick. It is on this animal, as might be expected, that the greater 

 part of the work on embryo- 

 nic growth has been done. 

 Hasselbalch, in the course of 

 his work on the respiration 

 of the chick embryo, ob- 

 tained a regular series for a 

 race not given. These corre- 

 sponded well enough with 

 the earlier data of Falck (also 

 from an unknown breed), 

 which were the first to be 

 published, appearing in 1857. 

 Hasselbalch's curve is shown 

 in Fig. 21, in which for the 

 first time we see the usual 

 ' ' embryo-placenta relation ' ' 

 in the form of a weight of 

 extra-embryonic structures 

 larger than the embryo in the 

 earliest stages, but soon falling below it.^ 

 relation between the two as follows: 



4-0 



C3.0 



2'0 



1«0 



O Membranes 

 • Embryo 



Hasselbalch calculated the 



See also Fig. 521. 



