UNION OF SPERM-CELL AND GERM-CELL. 317 



cell, whatever it may be, cannot be of that elementary indis- 

 pensable nature which is implied in the comparison of an 

 acid uniting with a base to form a salt. No alkali spon- 

 taneously develops into a salt ; without the acid the alkali is 

 powerless to assume any of the saline forms. But the germ- 

 cell does develop an embryo without the aid of a sperm-cell ; 

 and this, too, in certain animals which at other times generate 

 sperm-cells. Indispensable the influence of the sperm-cell 

 is, in the more complex organisms (although the insect is a 

 very complex organism) ; but we observe one intensely sig- 

 nificant fact, namely, that the germ-cell spontaneously j^f^sses 

 through the same early phases of its development, whether 

 it be fertilised or not. It cannot continue its development, as 

 the germ-cells of Polypes, Entomostraca, Bees, and Moths con- 

 tinue theirs ; but neither is there any fixed limit to its arrest. 

 Some ova fall short at one stage, others at others, but at no 

 stage of their history can we say, Here the aid of fertil- 

 isation begins. Every ovum, therefore, of the highest ani- 

 mal as of the lowest, has within it the power of development 

 unaided by the spermatozoon ; this development falls very 

 short indeed of an embryo, in the highest animals, but it 

 travels some miles on the road towards that goal ; and when, 

 as in insects, the goal is not very distant, it may be reached. 

 We may liken the spermatozoa to the extra pair of horses 

 put to the carriage to enable it to reach a certain distance 

 over mountainous oround. Two horses have drao-ffed the 

 carriage to the foot of the hill, and have brought it by 

 precisely the same route as tlie four horses would have 

 taken ; but here, at the foot of the hill, the extra horses are 



