FERTILISATION 219 



ment without developing a membrane such as is invariably 

 formed in normal eggs shortly after the entrance of the sper- 

 matozoa. Secondly, the rate of development in the artificially 

 fertilised eggs was considerably slower than in the eggs fertihsed 

 by spermatozoa. Thirdly, the larvse arising from osmotic 

 parthenogenesis, as soon as they began to swim, did so at the 

 bottom of the dish in which they were placed, instead of rising 

 to the surface of the water hke normal larvse. It was found 

 also that the percentage of eggs which could be induced to 

 develop by the osmotic process was invariably very much 

 smaller than the percentage of normally fertihsed eggs which 

 underwent development. The consideration of these differences 

 led Loeb to conclude that the spermatozoon in normal fertilisa- 

 tion carried into the ovum not one, but several substances or 

 conditions, each of which was responsible for a part only of 

 the normal characteristics of the process ; and that, in order to 

 imitate successfully the action of the sperm, it would be necessary 

 to combine two or more artificial methods. 



When the eggs of Strongylocentrotus purpuratus were put into 

 50 cubic centimetres of sea-water to which 3 c.c. of a .deci- 

 normal solution of a fatty acid had been added, and were left 

 in this water for about a minute, and were then transferred to 

 ordinary sea-water, they were observed to form membranes. 

 It was also noticed that the eggs underwent internal changes 

 characteristic of nuclear division, but they were rarely seen to 

 segment. Subsequently they began to disintegrate, and after 

 twenty-four hours were nearly all dead. If, however, the ova, 

 after they had formed a membrane, were deposited in sea- 

 water which had been rendered hypertonic by adding 15 c.c. 

 of sodium chloride solution of two and a half times the normal 

 concentration, to 100 c.c. of sea-water, aU or nearly all the eggs 

 could be induced to develop. Furthermore, the rate of de- 

 velopment was practically the same as that of normally fertihsed 

 eggs, a large percentage of the blastulse looked normal and rose 

 to the surface of the water, and the plutei which developed 

 showed the usual degree of vitaHty. 



The brothers Hertwig ^ had previously discovered that sea- 



' Hertwig (0. and K.), Untersiichungen zur Morphologic und Physiologie 

 der Zelle, Jena, 1887. 



