124 



For embryology. — In obtaining insect eggs it is highly 

 desirable, often imperatively necessary, to know the exact 

 age of the specimens. Hence it is desirable to capture 

 females of the species to be studied, and if possible have the 

 eggs laid in captivity. Thus the exact time of oviposition, 

 and consequently the age of the specimens studied, can be 

 known. For work simply illustrative of the enibryologic 

 development of insects, eggs should be chosen which are 

 large, and are not spherical, but of such shape that the speci- 

 mens can be readily oriented at the time of imbedding or 

 cutting. 



PREPARATION OF MATERIAL FOR SECTIONING. 



The sequence of treatment. — The fresh, live mate- 

 rial (bits of tissue, organs, whole insects, or eggs), which is to 

 be studied by means of sections, must undergo certain treat- 

 ment necessary to preserve it, and to prepare it for cutting. 

 It nas first to be killed d^wd fixed ; next to be hardened ; next 

 to be cleared ; and finally to be infiltrated with and imbedded 

 in paraffin (or collodion, see note later). (See also Recapit- 

 ulation, p. 138.) 



Killing and fixing. — Out of the bewildering array of 

 fixing agents, the student of insect histology must choose a 

 few which are characterized by gr'eat penetrating power. 

 Many of the commonly used agents for fixing animal tissues 

 in general cannot be used in work with insects (except where 

 dissected-out, non-chitinous organs or tissues are being han- 

 dled), because of their inability to penetrate the chitinized 

 integument. 



In fixing, a relatively large amount of the fixing agent 

 should be used, say at least 50 times the volume of the 

 specimens. The objects should sink, if not at first, then 

 later, in the fixing fluid. If the objects persist in floating 

 they probably contain air, and the tissues adjoining air bub- 



