4IO Ralph S. Lillie 



normally precedes the solution of the nuclear membrane of the 

 immature egg, in some manner inhibits the oxidations on which 

 this change depends. Just why this effect should result remains 

 for the present obscure; possibly several distinct chemical proc- 

 esses are concerned, having different coefficients of acceleration 

 by rise of temperature; at the higher temperature the available 

 oxygen may enter into a quite different reaction from that on 

 which the maturation-change depends; the latter would then be 

 prevented through a deficiency of available oxygen. Oxidations 

 in one set of processes may easily involve reductions in another 

 if the supply of free oxygen is Hmited. What is remarkable is 

 that maturation is prevented permanently by warming at this 

 stage. Warming after the germinal vesicle has broken down has 

 no effect on the course of maturation, the polar bodies forming in 

 the usual manner; and after this process is complete the eggs, as 

 already seen, may proceed to cleave and develop without fertiliza- 

 tion. Apparently conditions unfavorable to maturation produce 

 a permanent prevention of the process only if they act during the 

 brief period immediately following the deposition of the eggs; this 

 is for some season a critical stage, and if the maturation process 

 is not then begun it fails altogether. In harmony with this inter- 

 pretation is the well known fact that starfish eggs which show no 

 signs of maturing by twenty minutes or so after removal from the 

 animal to normal sea-water remain immature permanently. 



The effects of momentary warming at stages succeeding the disso- 

 lution of the germinal vesicle vary, as just shown, according to the 

 exact period at which the treatment is applied. As already seen, 

 membrane-formation and development may result from warming 

 very soon after the vesicle begins to lose its distinct outline. The 

 conditions are at first unfavorable, only a small proportion of eggs 

 forming membranes, and still fewer developing to a free-swimming 

 stage. In general, as indicated by Table IV, the proportion of 

 favorably developing eggs shows a progressive increase until an 

 optimum stage is reached — usually about 15 or 20 minutes before 

 the separation of the first polar body; warming at the time of 

 separation of this polar body rarely results in larvae, and in later 

 stages the conditions become steadily less favorable with lapse 

 of time. 



