Section VI 



CHAPTER 4 



Insects 



DIETRICH BODENSTEIN 



Progressive differentiation is the process by 

 which the fertilized insect egg is transformed 

 into the complete embryo through develop- 

 mental events closely coordinated in time 

 and space. The initiation and progress of 

 organization in the egg depend upon the 

 realization of certain dynamic phenomena 

 which mold the embryonic material for par- 

 ticular functions. It is the special physico- 

 chemical nature of the insect egg that pro- 

 vides the substrate and conditions for the 

 action and interaction of these dynamic 

 forces. 



THE INITIATION OF EARLY EMBRYONIC 

 ORGANIZATION 



The insect egg is usually rich in yolk. Its 

 nucleus lies in a central position and is 

 embedded in a small cytoplasmic island. 

 Fine cytoplasmic strands of this island ram- 

 ify through the yolk and often condense at 

 the periphery of the egg, forming here a 

 cortical layer. After the nucleus has di- 

 vided and its daughter nuclei have populated 

 the yolk, the majority of them move toward 

 the egg surface. Here they arrange them- 

 selves with the formation of cell boundaries 

 into a single cell layer, the blastoderm. The 

 first visible differentiation of the embryo is 

 the germ band. It appears in the blastoderm 

 in the region of the presimiptive prothorax 

 and from here continues its differentiation 

 anteriorly and posteriorly. These beginnings 

 of organization in the egg are governed by 

 two different centers, the activation center 

 and the differentiation center (Seidel, '29). 

 Through an alternation of dynamic processes 

 and material reactions, these two centers 

 interact, thus setting into motion the whole 

 process of embryonic organization. 



The activation center is located at the pos- 

 terior pole of the egg. Its function depend'; 

 on the interaction of the cleavage nviclei with 

 some factor in the region of the center. 

 The product of this reaction, presumablv 



a specific material substance, spreads for- 

 ward in the egg, evoking in its covirse an- 

 other reaction which changes the structure 

 of the yolk system. This in turn causes a 

 contraction of the yolk system. The latter 

 reaction provides the necessary situation for 

 the aggregation of the blastoderm cells to 

 form the germ band. 



The contraction begins at the site of the 

 presumptive prothorax and spreads from here 

 anteriorly and posteriorly in a wavelike fash- 

 ion. This region is known as the differentia- 

 tion center; in this same region the first 

 visible differentiation of the germ band may 

 be witnessed. The morphologically defined 

 differentiation center hence can be visualized 

 as a center of morphodynamic movement 

 which provides the stimulus for the aggrega- 

 tion of the cells that form the germ band. 

 The chain of reactions evoked by the activa- 

 tion center is thus essential for the function 

 of the differentiation center, which repre- 

 sents the focal point for all subsequent proc- 

 esses of differentiation of the embryo. 



While the described phenomena do not 

 directly concern the formation of the blasto- 

 derm, it must be realized that the entire 

 specialized organization of the insect egg as 

 a dynamic system, including the yolk sys- 

 tem, the cleavage nuclei with their cyto- 

 plasmic connections, and the blastoderm, 

 is the prerequisite for the normal sequence 

 of the described reactions. 



DETERMINATION AND REGULATION 



The problem of regulation and determina- 

 tion is closely related to, and can only be 

 understood in the light of, the above dis- 

 cussed general principles of insect develop- 

 ment. It has been shown that the regulative 

 capacity of the egs, varies greatly within the 

 different groups of insects (Seidel, Bock, and 

 Krause, '40). As a matter of fact, all transi- 

 tions from eggs with great regulative powers 

 to eggs exhibiting strictly mosaic develop- 



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