PREFACE vii 



The sequence of the orders of insects given in the table of contents is 

 not in agreement with that usually adopted in textbooks, but it is felt 

 that it expresses a more natural grouping so far as this is possible in a 

 linear arrangement. In making this arrangement the authors have been 

 guided by the works of Crampton, Handlirsch, Imms, Lameere, Marty- 

 nov, and Tillyard as well as by suggestions from their colleague Prof. J. C. 

 Bradley. For obvious reasons generic and specific names used in the 

 text are those employed by the authors of the original articles. Only in 

 cases where confusion might result in the use of an older name is another 

 term employed. 



Although much work has been done on the embryonic development of 

 insects in general, an examination of the text and references reveal that 

 many problems, both theoretical and factual, have not yet been solved. 

 Future workers, however, are reminded of the words of Nelson, which are 

 still appropriate after twenty-five years: 



There is need of further investigation, particularly of the more generalized 

 types. Superficial study, however, would be worse than useless; the type of 

 investigation demanded is of the highest, requiring the delicate and precise 

 methods of the cj^ologist, the best fixation and staining possible, a complete 

 series of stages, a study of the origin of the rudiments cell by cell, and finally 

 an eye single to the facts and regardless of preconceived theoretical considerations. 



In conclusion it may be said that great activity in experimental 

 embryology recently manifested both in Europe and America has stimu- 

 lated interest in normal embryology. Therefore we may look forward to 

 further work on controversial questions that have arisen in the last 

 seventy years, and on the later development stages that involve the 

 formation of the exoskeleton, the individual muscles, and the sense organs. 



oskar a. johannsen, 

 Ferdinand H. Butt. 

 Cornell University, 

 Ithaca, N. Y., 

 January, 1941. 



