PART I 



CHAPTER I 



INTRODUCTION 



ONTOGENETIC DEVELOPMENT 



In its broadest sense the ontogenetic developmental history of an 

 animal not only embraces all stages from that of a fertiUzed egg (ovum) 

 to that of the sexually mature adult but includes also a period of senes- 

 cence. That a final stage of decadence must be included is obvious when 

 one reflects that even during the earliest (embryonic) period provisional 

 structures arise only to degenerate after functioning for a shorter or 

 longer time. We therefore recognize for the majority of animals four 

 periods of development, not sharply marked to be sure, that in general 

 are recognizable by certain external characteristics. 



The first period is that of embryonic development that begins with 

 the fertilized egg or the first cleavage stage and ends at hatching or at 

 the birth of the animal. The transition from this stage to the next is a 

 gradual one even among the Eutracheata which for the most part are 

 oviparous. For example the eggshell, or chorion, among the Myriapoda 

 and the Collembola and some other insects is ruptured some time before 

 the completion of the embryo, whereas in other cases a fully formed insect 

 larva may lie dormant within the eggshell without emerging for a period 

 of several months. 



The second, or adolescent, period is a period of growth, during which 

 the reproductive organs become mature. It is an interval of marked 

 change in the outward appearance of many animals, notably among 

 amphibians and hemimetabolous and holometabolous insects. Desig- 

 nated as the ''postembryonic period" in insects, it is here frequently 

 much prolonged, lasting in some instances for months or even years. 

 The third period is that of sexual maturity and reproduction. Among 

 vertebrates and some invertebrates it may extend through a number of 

 years, but among the Eutracheata the period is usually much shorter, 

 lasting in the case of the Ephemeridae but a few hours. The final stage 

 is that of senescence, or a period of decadence and death. Phylogeneti- 

 cally it has no significance. It is the first of these stages, the embryonic 

 period of development, with which we are here concerned. 



Among the ancients, numerous animals, including amphibians and 

 many insects, were thought to arise spontaneously, and it was not until 



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