THE VITAL PROPERTIES OF THE CELL 291 



observed in the organic kingdom during the process of fertilisa- 

 tion, we must now turn our attention to a still wider and more 

 difficult subject the examination of the properties which the 

 cells must possess in order to unite themselves in the reproduc- 

 tive act, and thus to constitute a starting point for a new cycle of 

 development. 



In the first instance it is evident, that not all the cells of a 

 multicellular organism are capable of fertilising or of becoming 

 fertilised, and that even the sexual cells are only suitable for 

 reproductive purposes for, in many cases, quite a limited time. 

 Hence definite characteristics must be developed in the cells ; 

 these we will provisionally group under the common name of 

 " need for reproduction." 



This need for reproduction alone is in itself far from sufficient 

 to ensure the occurrence of fertilisation. This is proved by the 

 fact that, if mature eggs and spermatozoa from different organ- 

 isms are brought together, they do not pair. Hence a second 

 factor is necessary : the cells which are to unite sexually must 

 suit one another in their organisation, and in consequence must 

 have the inclination to combine with one another. This we will 

 designate as sexual affinity. 



Thus the physiology of the process of fertilisation may be 

 separated into two parts : 



1. Investigation of the need for reproduction. 



2. Examination of the sexual affinity of the cells. 



In a third section various hypotheses, which have been started 

 by various investigators, concerning the nature and aim of fertili- 

 sation, will be investigated. 



1. The "Need for Reproduction " of Cells. By the expression 

 "need for reproduction," we understand a condition of the cell, 

 when it has lost the capacity of carrying on the vital processes by 

 itself, although it regains the power to a still greater degree after it 

 has fused with a second cell in the act of fertilisation. At present 

 we entirely lack a deeper insight into the nature of this condition ; 

 for it is one of the inherent properties of living matter, and as 

 such is outside of the domain of our perceptive powers, since these 

 properties can only be recognised by their results. Similarly the 

 physiological side of the subject is completely unknown, since it 

 as yet has not been subjected to systematic investigation. Hence 

 we can only here mention certain observations, which must be 

 extended and widened in future by means of physiological investi- 



