2 8o CAUSES OF VARIATION 



5. All experiments indicated that it is impossible to secure 

 more than the beginning of segmentation from an unfertilized 

 egg without raising the concentration of the sea water. 



6. But for this purpose MgCl 2 was peculiarly effective and 

 normal plutei (free-swimming larvae) developed from unfertilized 

 eggs lying in normal sea water after having lain for two hours 

 in a solution of MgCl 2 of proper strength. 1 



7. These effects seemed to be due to the increased concen- 

 tration in the sea water, bringing about increased osmotic pres- 

 sure and resulting in a loss of water on the part of the egg. 

 This loss of water seems to be the active cause of rapid seg- 

 mentation, and a variety of substances were discovered which 

 were able to bring it about. 



8. The principal difference noticeable in the plutei was that 

 those developed from fertilized eggs swam freely at the top of 

 the water, while those developed from unfertilized eggs " were 

 all at the bottom of the dish and unable to rise." 



9. Experiments upon the marine annelid Chaetopterus 2 indi- 

 cated that artificial development is easier than with the sea 

 urchin, but that it is achieved by a different solution. In the 

 words of the experimenter, " We may say that Chaetopterus 

 possesses a higher degree of parthenogenetic tendency than the 

 Arbacia [sea urchin] eggs," 3 and "if the sea water contained 

 only a slightly greater proportion of K, we should find that 

 Chaetopterus was normally parthenogenetic." 4 



10. If certain forms are prevented from becoming partheno- 

 genetic by the constitution of the sea water, we may infer that 

 those which are naturally parthenogenetic are so by the consti- 

 tution of the blood or the sea water enabling the egg to develop. 5 



11. "The bridge between the phenomena of natural and 

 artificial parthenogenesis is formed by those animals in which 



1 Loeb, Studies in General Physiology, Part II, p. 624. 



2 " Mead had already found that if 0.5 per cent KC1 is added to sea water the 

 unfertilized eggs of Chaetopterus throw out their polar bodies, while the addition 

 of 0.5 per cent NaCl produced no such effect." Loeb, Studies in General 

 Physiology, Part II, pp. 656-657. 



8 Loeb, Studies in General Physiology, Part II, pp. 654-655. 

 4 Ibid. p. 665. 

 Ibid. p. 683. 



