CELLS AND NUCLEL 263 



changes which occur in the pupa, and before giving a detailed 

 account of the various organs of the imago. 



Upon making a careful examination by the aid of the micro- 

 scope, it will be found that the number of distinct forms of 

 texture or tissue is comparatively small, and, although many 

 differences may be observed in different parts, a study of the 

 transitional forms shows that all the tissues may be classed 

 under one or other of the following heads : 



1. Epithelial tissues. 



2. Amoeboid cells. 



3. Connective tissues. 



4. Muscular tissues. 



5. Nervous tissues. 



When any tissue is examined with the microscope, it 

 may be resolved into a number of parts, which are repeated 

 over and over again. These parts are termed tissue-elements, 

 and many tissues retain certain structural elements which are 

 termed cells, and which closely resemble the primary cell from 

 which the whole body is developed. Others, as the muscles 

 and nerves, although developed from cells, soon become so 

 modified that their cellular character is not apparent at first 

 sight, but a careful study shows indications of their cellular 

 origin. 



In addition to the cellular constituents, many of the tissues 

 exhibit a more or less homogeneous intercellular substance, 



133. Rath, Otto Von, ' Hautsinnes-organe der Insecten.' Zeitsch. f. w. 

 Zool., Bd. xlvi., 1888. 



134. BuTSCHLl UND ScHEWl AKOFF, ' Ueber der feineren Bau der 

 quergstiiften Muskeln der Anhropoden.' Biol. Centralblatt, Bd. xi., 

 P- 33. 1891. 



135. SCH.AI'ER, E. A., ' On the Minute Structure of the Muscle-Columns, or 

 Sarcostyles, which form the Wing-Muscles of Insects.' Proc. Roy. 

 See. Lond., vol. xlix., p. 280, 1891. 



136. SCHAFER, E. A., ' General Anatomy or Histology.' Quain's 

 ' Anatomy,' edit, x., vol. i., pt. 2., published separately. London and 

 New York, 1891. 



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