SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 725 



combustion of a certain amount of solid. Hanan, in fact, in some 

 unpublished experiments, has noticed a close relation between 

 humidity of environment and size of air-space. 



Another interesting corollary of the respiration of the embryo 

 was brought out by Hammett & Zoll. These workers studied the 

 response of the vitelline vessels of the chick embryo to various chemical 

 stimulants, by injecting them into the yolk with a micropipette in 

 the immediate vicinity of a length of vessel observed through a 

 microscope. In this way they ascertained that the walls of the 

 vitelline vessels are sensitive neither to the H ion nor to the OH ion, 

 changes in their concentration varying from pH 5-0 to 9-0 provoking 

 no alterations in the calibre of the vessels. On the other hand they 

 are specifically reactive to carbon dioxide and the invariable re- 

 sponse was one of constriction — moreover, the effective agent was not 

 HCO3 but either carbonic acid or carbon dioxide, for hydrochloric 

 acid solutions saturated with the gas were as efficient as water 

 saturated with it^. What follows, or may follow, from these results, 

 was thus suggested by Hammett & Zoll. It is obvious that tempera- 

 ture variations during incubation under the hen would induce 

 variations in the production of metabolites from which the embryo 

 builds its tissues. With rising temperature this would be accelerated 

 and so would carbon dioxide production. The latter by its constrictive 

 action on the vitelline vessels would cut down the blood-supply to 

 the embryo and thus prevent its being flooded with more food than 

 it could profitably handle. With falling temperature the processes 

 would be reversed, and the relaxed vessels, carrying a larger 

 volume of blood of lesser metabolite content, would thus provide 

 the embryo with adequate material for uninterrupted development. 

 It is conceivable that the regulation of the food-supply is controlled 

 in part by the COg-sensitivity of the blood-vessels of the yolk-sac. 

 Hammett & Zoll also applied their views to the process of inclusion 

 of the yolk-sac within the embryonic body at the end of incuba- 

 tion, and suggested that the large amounts of carbon dioxide then 

 being evolved constricted the blood-vessels of the yolk-sac to so 

 great an extent as to cause the atrophy which normally occurs. 



^ This was confirmed by Lange. The vascular membranes contain no nerve fibres 

 (Lange, Ehrich & Cohn) and elastic fibres are not to be found in their vessels (Cohn 

 & Lange). The capillaries are more irritable than the arterioles, and at the end of 

 development there is no degeneration ; the vessels die in complete possession of their 

 physiological irritabiliiy and anatomical integrity. 



