114 EXTERNAL FACTORS III. 6 



we have seen, that it is somewhat accelerated. At a pressure of 

 2^ atmospheres, however, segmentation is arrested and death 

 ensues (Samassa). 



When the tadpoles, newly hatched, are exposed to its influence, 

 the hyoid becomes immensely thickened and the branchial 

 chamber completely closed ; the internal gills are weak (Rauber), 

 and the same author states that l gastrulae ' subjected to three 

 atmospheres of ordinary air had their development temporarily 

 arrested, while later embryos, in the stage of the medullary folds, 

 became small and immobile in air at twice the atmospheric pressure. 

 This result seems to be due to pressure, not to the oxygen. 



In the foregoing the general necessity of respiration for the 

 life of the developing organism has alone been taken into con- 

 sideration, but it should not be forgotten that oxygen may 

 exert a stimulus on some part, the response to which results in 

 a process of differentiation. Thus His has suggested that the 

 growth of the blastoderm over the yolk is oxygenotropic, and 

 Herbst that the migration of the blastoderm-forming cells to 

 the surface in Arthropod ova, and the migration of spicule- 

 forming cells in Echinoid larvae are cases of definite reaction 

 to oxygenotactic stimuli. Loeb, we may note, has found that 

 the regeneration of the head of Tubular ia will only take place 

 when the stem is supplied with fresh water, and the same author 

 has suggested that the accumulation of the pigment cells round 

 the blood-vessels on the yolk-sac of F undid us is also an 

 oxygenotaxis. 



LITERATURE 



C. FERE. Note sur 1'influence des enduits partiels sur 1'incubation de 

 1'ceuf de poule, C. B. Soc. Biol. (10) i, 1894. 



C. GIACOMINI. Influence de 1'air rarifie sur le developpement de 

 1'oeuf de poule, Arch. Ital. Biol. xxii, 1895. 



E. GODLEWSKI. Die Einwirkung des Sauerstoffes auf die Entwicklung 

 von Eana temporaries, Arch. Ent. Mech. xi, 1901. 



C. HERBST. Ueber die Bedeutung der Reizphysiologie fur die 

 caueale Auffassung von Vorgangen in der thierischen Ontogenese, 

 Biol Centmlbl. xiv, xv, 1894, 1895. 



J. LOEB. Ueber die relative Empfindlichkeit von Froschembryonen 

 gegen Sauerstoffmangel und Wasserentziehung in verschiedenen Entwick- 

 lungsstadien, Pflugers Arch. Iv, 1894. 



