274 THE CELL 



less dense and does not become so strongly stained as the head of 

 the spermatozoon. 



This difference in size and consistency soon disappears during 

 the course of the process of internal fertilisation ; for the sperm- 

 nucleus, which was at first small, whilst on its way to the egg- 

 nucleus, soon swells up to the same size as the latter by absorb- 

 ing fluid out of the yolk (Fig. 142 JI), as is seen in the eggs of 

 most Worms, Molluscs, and Vertebrates. It is true that in iso- 

 lated cases, as in the eggs of the Sea-urchin (Fig. 141), the nuclei 

 are of different sizes, when they unite ; under these circumstances 

 the sperm-nucleus has taken up a smaller quantity of sap than 

 usual, and is consequently somewhat denser in consistency; so 

 that, in spite of the difference in size, we may still assume that 

 an equal amount of solid active constituents is present in both. 



It may be demonstrated in suitable objects, that the relative 

 size of egg and sperm-nuclei depends chiefly upon the time at 

 which the egg- cell was fertilised, whether before, during, or after 

 the formation of the polar cells. For instance, if spermatozoa be 

 brought into contact with, an egg of Aster acanthion whilst the 

 polar cells are developing, the sperm-nucleus must remain for a 

 considerable time in the yolk before fusion commences, and in 

 consequence it swells up during this period by absorbing nuclear 

 sap, until it is of the same size as the egg-nucleus, which develops 

 after the second polar cell has separated off. On the other hand, 

 if fertilisation occurs after the egg-cell is provided with both the 

 polar cells and the egg-nucleus, the sperm-nucleus remains for 

 only a few minutes as an independent body in the yolk, com- 

 mencing almost immediately after its entrance to fuse with the 

 egg-nucleus. Under these circumstances it keeps small in size, for 

 it is not able to saturate itself in the same way with nuclear sap. 



Thus we may consider the following important law as proved, 

 i.e. that the two sexual cells, in spite of the fact that frequently 

 they vary considerably in appearance and contain such unequal 

 quantities of protoplasm, contribute equal amounts of nuclear 

 substance (nuclein, in a definite number of nuclear segments, 

 paranuclein, in the ovocentrum and spermcentrum) during the 

 process of fertilisation, and in so far are equivalent. 



From this law I deduce the following : the nuclear substances 

 which are derived in equal quantities from two different indi- 

 viduals are invariably the only active substances, upon whose 

 union the act of fertilisation depends ; they are the true fertilisa- 



