The maturation and fertilization of the egg of Cerebratulus. 429 



and the chromosomes are arranged in an equatorial plate. I can 

 conceive of no way of determining beforehand which of the two 

 poles of the amphiaster is to be directed toward the periphery and, 

 therefore, lost in the polar body, and can only believe that it is due 

 to some chance difference in the relative position of the asters at an 

 early stage of the formation of the spindle. If there is any individual 

 difference in the chromosomes, any slight advantage in the position 

 of one aster or the other may be of the greatest influence on the 

 character of the future individual. These processes take place very 

 rapidly, so that the first polar spindle is fully formed and in its 

 definite position within twenty minutes after the egg is placed in 

 water. 



The egg develops up to this point whether it is fertilized or not, 

 and the rapidity of the process seems likewise independent of the 

 entrance of a spermatozoon into the egg. If the egg is not fertilized, 

 it remains in this condition for several hours, and eventually de- 

 generates. In eggs which have lain a very long time in water, the 

 entrance of a spermatozoon does not cause that sudden reaction of 

 the egg- cell which is apparently characterized in the fresh egg by the 

 formation of a delicate membrane impervious to other spermatozoa. 

 Therefore such eggs are entered by several spermatozoa, and develop- 

 ment is irregular. They seem to have much less vitality, and are 

 consequently less irritable, as O. and R. Hertwig (10) and others have 

 found to be the case when eggs are treated with dilute poisons or 

 subjected to an abnormally high temperature. If a vitelline membrane 

 is thrown off at the moment of the entrance of the spermatozoon, it 

 is so exceedingly delicate in these eggs, and lies so closely in contact 

 with the cell-substance that it is visible only at the point where the 

 polar bodies are formed, and ordinarily cannot be distinguished at 

 all. It thus happens that the polar bodies are usually lost during 

 the process of cleavage, and the difficulty of following out the cell- 

 lineage of the embryo is thereby greatly increased. 



If the egg has been fertilized during the formation and rotation 

 of the first polar spindle the slight indentation of the surface of the 

 egg at its outer pole, as noticed above, disappears, and in its place 

 a corresponding elevation is seen. The aster near the periphery of 

 the egg diminishes in size and distinctness, while that of the opposite 

 pole of the spindle becomes larger. The elevation of the surface in- 

 creases in height until it forms a conspicuous projection, and into 

 this the aster of the outer pole of the spindle passes, losing in its 



