CHAPTER IX 



SOME OF THE GENERAL CONSIDERATIONS RELATING 

 TO THE EMBRYOLOGY OF THE VERTEBRATA 



IN the course of the preceding chapters many of the general principles 

 of vertebrate embryology will have made themselves apparent : the 

 present chapter will deal shortly with some others of these principles 

 which seem to require special notice. 



(1) THE ONTOGENETIC EVOLUTION OF THE ZYGOTE INTO THE 

 COMPLETELY FORMED INDIVIDUAL. The Vertebrate commences its 

 individual existence as a zygote a single cell in which the 

 specific characteristics, derived from the paternal and maternal 

 ancestors, are already present though not recognizable. That 

 this latter statement is accurate is demonstrated by such a fact 

 as the following. The pelagic fertilized eggs of different species 

 of Teleostean fishes show no trace of the specific features which 

 characterize the adults. Such distinguishing features as are present 

 and enable a specialist to identify them are mere differences in 

 size, amount of yolk, colour of oil globule and so on, and have 

 nothing to do with adult characteristics. Yet if a selection of such 

 eggs are allowed to develop together under a homogeneous set of 

 environmental conditions each is found gradually to unfold the 

 complete array of characteristics which distinguish its own kind. 

 As the various zygotes have developed under the same identical set 

 of environmental conditions it follows that the differences which 

 gradually become apparent cannot be due to the moulding influence 

 of ext -rnal conditions: they must have been already present though 

 in invisible form in the zygote. 



It follows 1'u rt her that the evolution of the zygote into the adult 

 is in the main not a process of acquiring greater and greater c< in 

 plexity, in tli> of acquiring new properties, but ratlu-r if tin- 



localization tin- s.-^ rogation of special peculiarities in particular 

 portions of the, individual, so that these portions assume a specific 

 <li.nact.-r and become n-eo^ni/;ibl- as definite, tissues or organs. 

 The peculiarities were ilieiv to ln-jin \\ith. but they we re diffuse and 

 therefore UnieOOgllizable .^Mn.-\\hat in the same way as the various 



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