PAR THENOGENESIS 



cleave irregularly, both as to tempo and as to size, and they 

 tend to fall apart. In the next place, normally the fertilized 

 egg develops into a form which swims at the surface of the 

 sea-water; subsequent to treatment with this hypertonic 

 sea-water, those eggs which happen to develop as far as 

 the larval stage, never become top-swimming forms. 

 Finally, if sea-urchin eggs are in best physiological condition 

 at the time of fertilization, close to one hundred per cent, 

 of them reach the larval stage; on the other hand, with the 

 osmotic method of initiating development, the percentage 

 of developing eggs lies far below that obtained from fer- 

 tilized eggs of the same female; if the worker is careless, 

 using eggs which are not in best condition, he may obtain 

 no developing forms. 



Because Loeb noted that the vitelline membrane did not 

 separate, and that the larvae failed to swim at the surface 

 of the sea-water, he next endeavored to find a method which 

 would overcome these deficiencies. The result of his 

 investigations was the so-called improved method of arti- 

 ficial parthenogenesis for sea-urchins' eggs. This is the 

 famous fatty acid plus hypertonic sea-water method, some- 

 times called the Ivsin-corrective factor method. In this 

 method the fatty acid mostly employed is butyric acid. 

 Loeb first studied the eggs of the California sea-urchin, 

 Strongylocentrotiis purpuratus. He placed unfertilized eggs 

 in a solution of 50 cc. of sea-water plus 2.8 cc. of 1 10 normal 

 butyric acid and left them in this solution for one and one- 

 half to two and one-half minutes. On removal of these eggs 

 to normal sea-water, they separated membranes. Eggs 

 removed earlier than the minimum time did not separate 

 membranes nor did those eggs which remained in the acid 

 solution longer than optimum time, because prolonged 

 exposure to the acid is injurious. After the eggs had been 

 in normal sea-water for fifteen to twenty minutes, they were 

 immersed in a solution of 50 cc. of sea-water plus 8 cc. of 



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