FERTILIZATION AND CLEAVAGE 95 



mens (Sobotta, 1895; Huber, 1915; and others) that mam- 

 malian ova show no appreciable increase in size until the 

 blastocyst stage. 



The most convenient approach to the study of the physio- 

 logical processes underlying segmentation has involved the 

 study of the respiratory processes (Warburg, 1908-14 ; J. Loeb 

 and Wasteneys, 1912-15; J. Loeb, 1913; Runnstrom, 1930; 

 Whitaker, 1933; and others). Mammalian ova are available 

 in such small numbers that exact quantitative measurements 

 of respiratory activity are difficult to make and have not 

 been made. Nonetheless some indication of the nature of 

 the underlying processes may be had by the use of specific 

 poisons known to combine with and inhibit the reactions 

 of definite components of the chain of reactions involved in 

 respiration. Thus HCN is known to combine with iron- 

 containing enzyme phaeohemin which is the initial activator 

 in the aerobic phaeohemin-cytochrome chain (Warburg, 

 1932) and so to inhibit the respiration involving phaeo- 

 hemin activity. Cyanide also inhibits the cleavage of ova of 

 non-mammalian forms (Lyon, 1902; J. Loeb, 1906; see 

 Needham, 1932), as does an oxygen-free medium (J. Loeb, 

 1895). Runnstrom (1935) has demonstrated that the mitotic 

 process at segmentation in sea-urchin eggs is not dependent 

 upon the level of respiration since the addition of pyocyanine 

 to cyanide-inhibited egg suspensions restored oxygen con- 

 sumption to normal levels but no division ensued. 



Rabbit ova presumably develop in a medium relatively 

 low in oxygen, since the oxygen tension of the abdominal 

 cavity, and by inference that of the tubes (which have free 

 access to abdominal fluids), is 40 mni. Hg (Campbell, 1924) 

 as compared with 150 mm. Hg, the oxygen tension of the 

 air. It is of interest to inquire whether the segmentation of 

 rabbit ova is Hnked with the aerobic phaeohemin system. 

 Pincus and Enzmann (19366) have added KCN in appropri- 

 ate concentration to cultures of cleaving rabbit eggs and the 

 segmentation has ceased. Cinematographs of these ova 

 indicated that the eggs were not ''killed" by the poison 



