CHAPTER XII 



EXPERIMENTAL EMBRYOLOGY 



DETERMINATION 



Development from egg to adult is physiologically a continuous 

 process regardless of the form changes involved. Nevertheless it is 

 convenient to treat the subject under two arbitrary subheads, the present 

 chapter being concerned primarily with development within the egg. 

 The second division would include the development after emergence from 

 the egg. 



Although experimental insect embryology is still in a very early stage 

 in comparison with experimental vertebrate embryology, it has reached a 

 point where the literature is sufficiently extensive and the results suffi- 

 ciently definite to make a resume in English seem desirable. 



Most of the important data on the embryonic stages are of recent 

 date. Historically the first experiments were those of Wheeler (1889) 

 and Megusar (1906) on gravitational effect by the inversion of eggs. 

 The first important experiments were those of Hegner on beetle eggs. 

 In 1908 he studied the effects of puncturing the egg and allowing part 

 of the contents to flow out, in 1909 the effects of centrifugal force, and in 

 1911 the e^ffects of kilUng regions of the egg with a hot needle. A lapse 

 of about fifteen years followed, during which the little experimental data 

 came from genetical and cytological investigations. Then, with the work 

 of Reith (1925), Seidel (1926), and Pauh (1927) began the series of experi- 

 ments that form the basis of experimental embryology of insects of today. 

 To Seidel and his colleagues we owe most of our knowledge of the vital 

 processes occurring in the insect egg. 



The following review in large part is taken from a paper by Richards 

 and Miller (1937). 



For our purposes we shall regard ^^ determination'^ as the process of 

 primary chemo-differentiation that "sets the course" along which a given 

 region of the egg is to develop, i.e., determines its fate. Determination 

 lays the invisible foundation for morphological differentiation. It may, 

 and probably always does, vary in degree, becoming increasingly more 

 strict as development proceeds, and so may also be expressed as "a 

 limitation of potencies." Differentiation has been aptly defined by 

 Schnetter (1934a) as "visible distinctive configuration." It is the 

 realization, as visible development, of the capacities of a region acquired 

 by earlier or simultaneous chemo-differentiation. 



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