Artificial Parthenogenesis ni Starfish Eggs 427 



were found by Delage in the case of Strongylocentrotus, as had 

 already been determined by Loeb; here the presence of oxygen in 

 the hypertonic solutions is favorable to development. We have 

 thus a striking contrast between the two forms in respect to the 

 part played by oxygen in the initiatory process. This contrast 

 cannot be explained at present; it can only be referred to deep- 

 seated constitutional differences between the two eggs. One fur- 

 ther consideration is suggested and should be emphasized here: 

 it must be recognized clearly that the physiological conditions 

 underlying the initiation of development — i. e., the bringing of the 

 egg into a condition in which it becomes capable of automatically 

 passing through its characteristic ontogenetic cycle — may be of 

 quite different nature from those on which the developmental proc- 

 ess itself depends. This is seen in the fact that notwithstanding 

 the contrast in the conditions of the initiatory process, both the 

 above eggs require the presence of free oxygen for their develop- 

 ment. Unexplained constitutional differences between species 

 play a part here, and we are not yet in a position for broad general- 

 ization. Nothing but further exact investigation of the conditions 

 of artificial parthenogenesis in eggs of different groups can be 

 expected to bring to light the fundamental conditions common to 

 the different types. For the solution of this problem a system- 

 atically inductive procedure seems safest at present. 



SUMMARY 



1 Momentary exposure of the eggs of Asterias forbesii, dur- 

 ing the early maturation period, to temperatures of 35° to 38° 

 results in the formation of typical fertilization membranes, fol- 

 lowed by the development of many eggs to a free swimming 

 larval stage. 



2 The favorable duration of exposure to the above tempera- 

 tures is very brief, with a well-defined optimum for each tempera- 

 ture; this optimum is approximately 70 seconds for 35°, 40 to 50 

 seconds for 36°, 30 seconds for 37°, and 20 seconds for 38°. A 

 very rapid rate of decrease in time of exposure with rise in tempera- 

 ture is thus indicated, a rise of three degrees above 35° apparently 



