CHAP. XXXVI.] 



METAGENESIS. 



527 



Fig. 255. 



side turned towards each other ; the swelling increases until a sort 

 of process is formed from each : these at length meet ; the walls 

 become fused, the cavities continuous, and the contents of the two 

 cells become mixed. From this admixture a new body, termed a 

 spore or sporangium, results, by the development of which the new 

 plant is formed. 



In the higher plants and in animals, dis- 

 tinct organs are set apart for the formation 

 of the sperm cells and germ cells. By the 

 action of the contents of the sperm cell the 

 ovum becomes impregnated ; and under fa- 

 vourable circumstances, often quite indepen- 

 dent of the parent, changes result which give 

 rise to the formation of the embryo from 

 which the adult animal is gradually deve- 

 loped. 



Now, either the perfect form of the being 

 may be attained by the gradual and progres- 

 sive development of the embryo, or several 

 distinct phases of existence may be passed 

 through before the creature reaches its per- 

 fectly developed form. This latter condition 

 is seen in many of the lower classes of 

 animals, and is familiar to us in the class of 

 insects ; it is, in fact, what we understand by 

 metamorphosis. 



Metamorphosis. In metamorphosis, it must be carefully borne 

 in mind, that it is the self-same embryo which passes through cer- 

 tain transitional stages or phases, and ultimately becomes the per- 

 fectly developed animal ; a condition essentially different from that 

 which we shall next consider under the term Metagenesis or alter- 

 nation of generations, in which successive generations of larval crea- 

 tures are produced from larvae without the occurrence of any fresh 

 generative act. Here, instead of one individual passing through 

 several transitional forms, an imperfectly developed creature pro- 

 duces a multitude of forms resembling either itself or the perfect 

 individuals from which the ovum was formed which evolved it. 



Metagenesis. In some animals, the embryo, instead of being- 

 developed into a form resembling that of its parents, only attains 

 a sort of larval condition, the offspring of which, however, return 

 to the perfect type, instead of assuming the character of their 

 larval parent. Now, between the fully-developed animals of one 



Conferva bipuuctata in the 

 actof conjuation,after Meyen. 

 The cells from contiguous fila- 

 ments approach each other, 

 and ultimately their cavities 

 coalesce. The oval spores, re- 

 sulting from the action of the 

 contents of one cell upon the 

 other, are seen in two of the 

 cells. 



