32 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



masses of protoplasmic matter, derived from different parts 

 of the same organism, or from two organisms of the same 

 species, and the single mass which results from the fusion 

 develops into a new organism. 



In the majority of cases, however, there is a marked mor- 

 phological difference between the two factors in the process, 

 and then one is called the ^nale^ and the other the female^ 

 element. The female element is relatively large, and under- 

 goes but little change of form. In all the higher plants and 

 animals it is a nucleated cell, to which a greater or less 

 amount of nutritive material, constituting a food-yelk^ may- 

 be added. 



The male element, on the other hand, is relatively small. 

 It may be conveyed to the female element by an outgrowth 

 of the wall of its cell, which is short in many Algm and JBioigi^ 

 but becomes an immensely elongated tubular filament, in the 

 case of the pollen-cell of flowering plants. But, more com- 

 monly, the pnjtoplasm of the male cell becomes converted 

 into rods or filaments, which usually are in active vibratile 

 movement, and sometimes are propelled by numerous cilia. 

 Occasionally, however, as in many Nematoidea and Arthro- 

 poda, they are devoid of mobility. 



The manner in which the contents of the pollen-tube 

 affect the embryo cell in flowering plants is unknown, as no 

 perforation through, which the contents of the pollen-tube 

 may pass, so as actually to mix with the substance of the em- 

 bryo cell, has been discovered ; and there is the same diffi- 

 culty with respect to the conjugative processes of some of the 

 Cryptogamia. But in the great majority of plants, and in 

 all animals, there can be no doubt that the substance of the 

 male element actually mixes with that of the female, so 

 that, in all these cases, the sexual process remains one of con- 

 jugation ; and impregnation is the physical admixture of pro- 

 toplasmic matter derived from two sources, wdiich may be 

 either different parts of the same organism, or different organ- 

 isms. 



The effect of impregnation appears in all cases to be that 

 the impregnated protoplasm tends to divide into portions 

 {blastomeres), which may remain united as a single cell-aggre- 

 gate, or some or all of which may become separate organ- 

 isms. A longer or shorter period of rest, in many cases, 

 intervenes between the act of impregnation and the com- 

 mencement of the process of division. 



As a general rule, the female cell, which directly receives 



