THEORY OF SEX — ITS NATURE AND ORIGIN. 



129 



In the bell-animalcule, which grows so commonly on the 

 water-plants of our ponds, a minute free-swimming unit, formed 

 as one of the results of repeated division, unites with a stalked 

 individual of the normal size. In the related Epistylis, Engel- 

 mann has described how an individual divides first of all into 

 two cells. One of these remains as such (like an ovum), the 



Vorticella, the Bell-animalcule,— a, the normal individual ; 

 b, its division into two ; c, the division accomplished ; 

 d, the further division of one of the halves into eight 

 small (male) units ; e, a minute individual uniting with 

 one of normal size. 



Other repeatedly divides (like a mother-sperm-cell) into numerous 

 minute units. One of these subsequently unites with the 

 undivided cell, and Engelmann does not hesitate to call the 

 different elements male and female. In some radiolarians 

 {e.g., Collozoiim), dimorphic spores — large and small — have been 

 described, although their history has not yet been fully traced. 

 Even in Foraminifera, as Schlumberger, De la Harpe, and H. 

 B. Brady have shown, a marked dimorphism may occur ; and 

 here again the distinction seems to lie between preponderant 

 anabolism and katabolism. 



As another illustration, it will be instructive to select the 

 case of volvox. In this colonial organism, which is best re- 

 garded as a multicellular protist, the component cells are at first 

 all alike. They are united by protoplasmic bridges, and simply 

 form a vegetative colony. In favourable environmental con- 

 ditions this state of affairs may persist, or be interrupted only 

 by parthenogenetic multiplication. When nutrition is checked, 

 however, sexual reproduction makes its appearance, and that in 

 a manner which illustrates most instructively the differentiation 

 of the two sets of elements. Some of the cells are seen 

 differentiating at the expense of others, accumulating capital 



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