34 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



begin with a few words about the absolute size of its eggs 

 and spermatozoa. All of you are familiar with the eggs of 

 birds and possibly of frogs ; these are abnormally large 

 eggs, on account of the very high amount of reserve 

 material they contain. The almost spherical egg of our 

 Echinus only measures about a tenth of a millimetre in 

 diameter ; and the head of the spermatozoon has a volume 

 which is only the four-hundred-thousandth part of the 

 volume of the egg ! The egg is about on the extreme limit 

 of what can be seen without optical instruments ; it is 

 visible as a small white point. But the number of eggs 

 produced by a single female is enormous and may amount 

 to hundreds of thousands ; this is one of the properties 

 which render the eggs of Echinus so very suitable for 

 experimental research ; you can obtain them whenever and 

 in any quantity you like ; and, moreover, they happen to 

 be very clear and transparent, even in later stages, and to 

 bear all kinds of operations well. 



The spermia enters the egg, and it does so in the open 

 water another of the experimental advantages of our type. 

 Only one spermia enters the egg in normal cases, and only 

 its head goes in, the tail is left outside. The moment 

 that the head has penetrated the protoplasm of the egg a 

 thin membrane is formed by the latter. This membrane is 

 very soft at first, becoming much stronger later on ; it is 

 very important for all experimental work, that by shaking 

 the egg in the first minutes of its existence the membrane 

 can easily be destroyed without any damage to the egg itself. 



And now occurs the chief phenomenon of fertilisation : 

 the nucleus of the spermatozoon unites with the nucleus of 

 the egg. When speaking of maturation, we mentioned that 



