DURATION OF EMBRYO DEVELOPMENT. 41 



us still further enlightenment with regard to this 

 elasticity, and also make intelligible the manner in 

 which the spermatozoa force their way in through the 

 vitelline membrane. By pressure, and by turning the 

 egg with the coverslip, one may be convinced of the 

 great elasticity of the vitelline membrane, which is 

 widely separated from the egg. A further chance ob- 

 servation gave me a clear idea of the very remarkable 

 consistence of this membrane. In the examination of 

 later stages, in which the already ciliated embryo 

 rotates inside the egg membrane, it happened that 

 through the pressure of the slip, the egg membrane 

 tore in one place, and there a part of the soft embryo- 

 body forced its way outwards, as it were a bag bursting. 

 When the pressure of the cover-slip was removed, the 

 part of the embiyo which had been pressed forward 

 was abstricted by the egg membrane. The rest of the 

 ciliated embrj'O made its way inside the egg mem- 

 brane again, and the rupture of the latter disappeared 

 so completely that not a trace of it was any longer to 

 be perceived. This remarkable and, one might almost 

 say, plastic activity of the vitelline membrane, explains 

 how it is that without the formation of a micropyle 

 the spermatozoon can force its way into the egg. 



