CHAPTER VI 

 DEVELOPMENT 



By a developmental process is meant a series of changes in 

 the course of which an organic system assumes a specific con- 

 figuration. The systems may be (i) the little fleshy bud which 

 forms at the place where the appendage of a crab has been broken 

 off ; (2) the " proud flesh " which forms in the scar of a wound 

 that is about to heal and (3) ova, spermatozoa, buds, spores, etc. 

 The corresponding specific configurations are (i) the regenerated 

 limb of the crab ; (2) the normal, vascular, muscle, nerve and 

 connective tissues which are formed when the wound heals up ; 

 and (3) the organisms into which the ova, spermatozoa, spores, 

 buds, etc., develop. In the following sections of this chapter 

 we shall restrict the discussion to the cases of fertilized ova which 

 develop into animal organisms, noting that the fundamental 

 principles expounded can be made, with the necessary qualifica- 

 tions, to apply to all other ways in which animals reproduce. 



69. ON ANIMAL LIFE-HISTORIES 



A life-history begins when some cell in the gonad (or elsewhere) 

 of the animal body divides so that a free gonidial cell is essentially 

 detached from the parental body and undergoes the process of 

 maturation (Section 8i«). The matured, gonidial cell may be 

 spawned, or emitted, from the body of the parent or it may come 

 to be lodged in some cavity of the parental body (such as the 

 uterus) but it is not then part of that body and it is only associated 

 with the maternal body to the extent that it receives nutritive 

 materials from the latter. 



Development begins with the maturation of this gonidial 

 cell. We consider the processes of maturation in the following 

 chapters, noting merely in the present place that it involves meiotic 

 cell divisions, in the course of which the nuclear materials of the 

 gonidial cell become rearranged and either a matured ovum, or 



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