THE CYCLE OF LIFE 389 



contain some nuclear element which is not in harmony 

 with the particular protoplasmic composition of the eggs 

 of the Edible Frog and the Common Toad. But if this 

 disharmonious element be destroyed by the Radium in- 

 fluence, the spermatozoon may act as a stimulus to develop- 

 ment which is, in a sense, parthenogenetic. In various 

 organs, it was noted that the surface or volume of the 

 nuclei was half the normal. 



Another side-light may be illuminating. In many cases 

 it is possible to effect artificial hybridisation, even between 

 types which are very far apart. A very striking instance 

 is that effected by Professor E. W. MacBride between the 

 eggs of the common heart-urchin (Echinocardium cordatum) 

 and the sperms of the common regular sea-urchin (Echinus 

 esculentus). The hybrid larvae, which showed both pater- 

 nal and maternal characters, lived for only eight or nine 

 days, but all Echinoderm larvae are delicate and difficult 

 to rear. The interest of the case is that the two parent 

 genera are so far apart. Professor MacBride points out that 

 Echinus and Echiiiocardium have been distinct since the 

 beginning of the Secondary epoch, and that their common 

 ancestor could not have lived later than a period which a 

 moderate estimate would place at twenty million years ago ; 

 yet the germ-cells of the two types will commingle so as 

 to produce a hybrid in which both paternal and maternal 

 characters are represented. 



No one can dream of dealing in a facile way with 

 development, which is one of the central mysteries of 

 life, but we wish to try to state two general ideas. 

 The first is that development is an active process of 

 self-expression. This may be illustrated by reference to 

 a very important event in development, namely, the out- 



