714: HANDBOOK OF PHYSIOLOGY. 



its chief manifestations, and see how far these seem to be dependent on 

 other forces in nature, and how connected with them. 



Life manifests itself by Birth, Growth, Development, Decline, and 

 Death; and an idea of life will most naturally arise by taking these 

 events in succession, and studying them individually, and in relation to 

 each other. 



When the embryo in a seed awakes from that state, neither life nor 

 death, which is called dormant vitality, and, bursting its envelopes, 

 begins to grow up and develop, it may be said that there is a birth. 

 And so, when the chick escapes from the egg, and when any living form 

 is, as the phrase goes, brought into the world. In each case, however, 

 birth is not the beginning of life, but only the continuation of it under 

 different conditions. To understand the beginning of life in any indi- 

 vidual, whether plant or animal, existence must be traced somewhat fur- 

 ther back, and in this way an idea gained concerning the nature of the 

 germ, the development of which is to issue in birth. 



The germ may be defined as that portion of the parent which is set 

 apart with power to grow up into the likeness of the being from which 

 it has been derived. 



The manner in which the germ is separated from the parent does not 

 here concern us. It belongs to the special subject of generation. 

 Neither need we consider apart from others those modes of propagation, 

 as fission and gemmation, which differ more apparently than really from 

 the ordinary process typified in the formation of the seed or ovum. In 

 every case alike, a new individual plant or animal is a portion of its par- 

 ent: it may be a mere outgrowth or bud, which, if separated, can main- 

 tain an independent existence; it may be not an outgrowth, but simply 

 a portion of the parent's structure, which has been naturally or artifi- 

 cially cut off, as in the spontaneous or artificial cleaving of a polyp; it 

 may be the embryo of a seed or ovum, as in those cases in which the pro- 

 cess of multiplication of different organs has reached the point of sepa- 

 ration of the individual more or less completely into two sexes, the 

 mutual conjugation of a portion of each of which, the sperm-cell and 

 the germ-cell, is necessary for the production of a new being. We are 

 so accustomed to regard the conjugation of the two sexes as necessary for 

 what is called generation, that we are apt to forget that it is only gradu- 

 ally in the upward progress of development of the vegetable and animal 

 kingdoms, that those portions of organized matter which are to produce 

 new beings are allotted to two separate individuals. In the least devel- 

 oped forms of life, almost any part of the body is capable of assuming 

 the characters of a separate individual; and propagation, therefore, oc- 

 curs by fission or gemmation in some form or other. Then, in beings a 

 little higher in rank, only a special part of the body can become a sepa- 

 rate being, and only by conjugation with another special part. Still 



