Reproduction and Development. 



45 



generation, and their offspring in turn reproduce in like 

 manner, without any union or fusion of ovum or sperm. 

 The same is true of the little water-fleas, or Daphnids ; 

 while in some kinds of rotifers fertilization is said never 

 to occur. It is a curious and interesting fact, which seems 

 now to be established beyond question, that drone bees are 

 developed from unfertilized ova, the fertilized ova pro- 

 ducing either queens or workers, according to the nature of 

 the food with which the grubs are supplied. Where, as in 

 the case of aphids and daphnids, fertilization occasionally 

 takes place, it would seem that lowered temperature and 

 diminished food-supply are the determining conditions. 

 Fertilization, therefore, generally takes place in the 

 autumn ; the fertilized ovum living on in a quiescent state 

 during the winter, and developing with the warmth of the 

 succeeding spring. In the artificial summer of a green- 

 house, reproduction may continue for three or four years 

 without the occurrence of any fertilization. 



Mention may here be made of some peculiarly modified 

 modes of reproduction among the metazoa. The aurelia 

 is a well-known and tolerably common jelly-fish. These 



Fig. 9. — Aurelia: Life-cycle. 



a, embryo ; b, Hydra tuba ; c, Hydra tuba, with medusoid segments ; d, medusa separated 

 to lead free existence. 



produce ova, which are duly fertilized by sperms from a 

 different individual. A minute, free-swimming embryo 

 develops from the ovum, which settles down and becomes 

 a little polyp-like organism, the Hydra tuba. As growth 

 proceeds, this divides or segments into a number of se- 

 parable, but at first connected, parts. As these attain their 

 full development, first one and then another is detached 

 from the free end, floats off, and becomes a medusoid 



