SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 



683 



20th day, after which it slowly declines, never falling, however, 

 below about a third of its maximum value. This curve is, of 

 course, a true measure of metabolic rate, and obviously fits in 

 well with the work of Parnas & Krasinska, and of Bialascewicz 

 & Bledovski. We here see for the first time a possible reconciliation 

 between the apparently conflicting behaviour of the metabolic rate 

 in various types of embryo. As far as can be ascertained at present, 

 the metabolic rate in the echinoderm and the amphibian embryo 

 rises during its ontogenesis, yet there is ample evidence, as we shall 

 see later, that the metaboHc rate of the avian embryo consistently 

 falls, at any rate from the 4th or 5th day of incubation onwards. 



1-0 

 ^0-9 



S3 0-7 



-0.6 



h 0-4 

 ^0-3 

 I 0-2 



o 



10 20 30 40 50 60 70 80 90 100 110 120 130 

 Days after fertilization 



Fig. 133. 



Possibly the curve for the heat-production of the toad embryo gives 

 the clue in suggesting that in all embryos there is a point at which 

 the metabolic rate is higher than at any other time. The investiga- 

 tions of the extremely early stages in the echinoderm egg which we 

 have been discussing have on this view revealed only the upward 

 stretches of this curve, while the work on the chick embryo, which it 

 must be remembered has completed its gastrulation before the egg 

 is laid at all, has shown us the descending part of the curve. In 

 the mammal, moreover, the curve for heat-production per gram per 

 hour follows a peaked course ; for instance, the work of Wood and 

 his collaborators has revealed very accurately the time at which this 

 takes place in the pig. Two points must not be lost sight of in this 

 discussion, firstly, that, for questions such as these, no hard-and-fast 

 line can be drawn between embryonic or foetal growth and post- 

 natal growth; the act of birth or hatching may be relatively 



