112 



EXTERNAL FACTORS 



III. 6 



It has been noticed elsewhere that this pigment is less abundantly 

 formed in darkness than in light, and Loeb has suggested that 

 light may promote oxidation. 



The ova of Echinoids also require oxygen from the beginning 

 of their development (Loeb). Without this element segmenta- 

 tion is impossible, or, if segmentation has already begun before 

 they are deprived of it, the blastomeres swell up and fuse. 

 According to Lyon the eggs of Arbacia are only sensitive to 

 a want of oxygen for from fifteen to twenty minutes after 

 fertilization. Vernon has shown that water saturated with 

 carbon dioxide and mixed in the proportion of 20 % or more with 

 sea-water is fatal to the development of these forms. 



Exact quantitative determinations of the oxygen absorbed 

 and the carbon dioxide excreted have been made by Godlewski 

 for the Frog and by Pott and Preyer for the Chick. The results 

 are shown in the tables annexed (Tables XIV, XV). 



TABLE XIV 



Showing the result of one experiment on the respiration of 

 the Frog's egg (Godlewski). 



It thus appears to be the very general rule that the egg begins 

 to respire at an early age. There is a case, however, Ascaris, in 

 which not only can the egg endure an atmosphere of nitrogen 

 or carbon dioxide or nitric oxide for prolonged periods and still 

 develop, but is actually killed by pure oxygen (at 2 atmospheres) 

 (Samassa). The adult worm, of course, is an endoparasite, and 

 Bunge has shown that it can manage to produce carbon dioxide 

 though denied access to free oxygen. 



The effect of pure oxygen has also been tried on various embryos. 

 In such an atmosphere (at ordinary pressure) the development of 

 the chick is normal, except that skin, allantois, limbs and anmi- 

 otic fluid are all very red with oxyhaemoglobin ; an excessive 

 amount of carbon dioxide is produced. The amount of this gas 



