Embryology. 117 



may be supposed to have their seat in these fundamental 

 elements. They make up all living matters, are the 

 bearers of heredity, and the real builders of the organ- 

 ism." 



Chapter X. 

 Embryology. 



The Scope of Embryology Ancient Embryology Harvey Bonnet and 

 the Preformationists Wolff and Epigenesis Von Baer Alterna- 

 tion of Generations The Influence of the Cell-theory Nature of the 

 Ovum Nature of the Spermatozoon Fertilization Maturation 

 The Mode of Development Germinal Layers Influence of Evolu- 

 tion Doctrine The Gastraa Theory The Recapitulation Doctrine 

 Substitution of Organs Experimental Embryology Theories of 

 Development. 



Embryology is the study of the early stages in de- 

 velopment. Its problem is the making the "becom- 

 ing " of the organism up to a vague point The Scope of 

 at which the specific characters begin to be Embryology, 

 well defined. This limit is determined rather by con- 

 venience than by logic, for embryology is really but a 

 part of that larger study which considers a living crea- 

 ture in its time-relations, and is concerned with the 

 breaking down in old age as well as with the building 

 up in youth. 



Embryological study has two main aspects: it is, 

 on the one hand, morphological, describing the form 

 and structure of the organism at successive stages from 

 the fertilized egg to the adult ; it is, on the other hand, 

 physiological, seeking to disclose the immediate vital 

 conditions which lead on from stage to stage. The 

 first task is obviously the easier, for at any stage the 

 developing organism may be killed, dissected, sectioned, 

 and photographed; the second task is beset with un- 

 conquered difficulties. 



This paragraph is too much like that on "the snakes 

 of Iceland", for there was, so far as we are aware, 



