HABITS, ETC., OP CEREBRATCJLtJS LACTEUS. 131 



duced by the lashing of the long tail. Althongh this appears 

 like a weak movement under the microscope, it must really 

 be very strong, for the head of the sperm is pushed in steadily 

 and quickly. 



The curve in the head of the sperm renders any rotary 

 motion impossible. As soon as it has entered the egg the 

 latter becomes more opaque than before, and the large trans- 

 lucent germinal vesicle quickly breaks down and disappears. 

 This is followed in turn by the giving off of the polar bodies. 



If the supply of sperm be sufficient, so many of them pene- 

 trate the membranes that the latter are broken down and 

 disappear entirely. Then the sperms, clinging to the egg 

 with their heads and lashing with their tails, give the egg a 

 rotary motion, exactly similar to that produced in later de- 

 velopment by the cilia. This may be kept up for an hour or 

 more. 



Formation of Polar Bodies. — The first polar body 

 appears about an hour and a quarter after fertilisation, and 

 always at a point diametrically opposite the protuberance in 

 the limiting membrane, when such a protuberance is present. 



As already noted, this is the remnant of the original stalk 

 whereby the ovum was joined to the wall of the ovary, and 

 hence those eggs which do not exhibit it must have been 

 separated so gradually and completely as to have left no 

 trace of their former connection. But it is reasonable to 

 infer that the polar bodies appear in them also opposite to 

 the point where such connection formerly existed. If this be 

 true, it follows that the connection of the ovum with the wall 

 of the ovary during development results in its permanent 

 polarisation, and the point at which the polar bodies are to 

 appear is predetermined. 



There is no perceptible separation of formative and nutri- 

 tive material, but correlated phenomena indicate that the 

 point of attachment is the vegetative pole. The food supply 

 must come to the ovum through this attachment, and in early 

 development the germinal vesicle often lies eccentrically near 

 it (fig. 64). These facts, taken in connection with what has 



