EMBRYOLOGY AND REPRODUCTTON. 69 



animal grows gradually out of the other, like a bud 

 out of a stem. In many forms there is an alternate 

 occurrence of sexual and asexual reproduction, pro- 

 ducing what is known as alternation of generations 

 (see p. 150). The asexual mode of development does 

 not occur among the higher animals; but among 

 the insects there is a variation of it called partheno- 

 genesis, in which new individuals are produced from 

 germs which are very like ova in appearance, but do 

 not require to be fertilized. It is believed that the 

 essential difference between true egg-cells and these 

 is that the former cast off certain small bodies formed 

 from portions of the protoplasm of the nucleus, and 

 called polar bodies. It is believed that in this way 

 the ova lose the power of growing on their own 

 account, and become dependent for further develop- 

 ment on the stock of energy which is contributed 

 to them by the male cell which they absorb. True 

 ova cast off two polar bodies. The asexual germs 

 above spoken of (pseudova, or false egg-cells), which 

 occur in the common Green-fly or Plant-louse (Aphis), 

 form but one polar body ; instead of casting off this, 

 they retain it within themselves. 



The egg-cell, which, as already stated, is a round 

 cell with a particularly large nucleus and nucleolus, 

 is fertilized by the male cell, which fuses with it much 

 as the free Vorticella fuses with the fixed one, only that 

 the discrepancy in size is greater. The egg- cell then 

 begins to divide by two, and again by two several 

 times, until a bunch of cells is formed. This process 



