THE GENERATIVE ORGANS. 659 



Insects frequently exhibit secondary sexual characters, as 

 differences of size, form and colour. In the Blow-fly the shape 

 of the forehead and position of the eyes varies in the two sexes 

 (see p. 120), and the males are somewhat smaller than the 

 females. 



The generative organs of both sexes may be classified in 

 three groups : (i) The generative glands or gonads, and their 

 segmental ducts ; (2) The secondary ducts ; and (3) The 

 external organs, which usually consist of one or more pairs of 

 abdominal appendages termed gonapophyses. 



(c} Suite, DeVeloppement de 1'CEuf pondre. Ann. Sc. Nat. Zool., 

 Ser. v., 1869-1870. (a) Tom. xi. ; (b} torn. xiv. ; (c) torn. xv. 



326. GRIMM, O., ' Die Ungeschlechtliche Fortpflanzung einer Chiro- 

 nomusart, und deren Entwickelung aus dem unbefruchteten Ei.' 

 Mem. Acad. Petersbourg, torn, xv., 1870. 



327. BRANDT, A., 'Ueber die Eirohren der Blatta (Periplaneta) Orientalis.' 

 Mem. Acad. St. Pdtersb., Bd. xxi., 1874. 



328. RAJEWSKI, ' Ueber die Geschlechtsorgane von Blatta Orientalis.' 

 Nachr. d. kais. Gesellsch. d. Moskauer Universitat, Bd. xvi., 1875. 

 Abstract in ' Hofmann and Schwalbe Jahresbericht,' 1875, p. 425. 



329. GERSTACKER, A., ' Die Klassen und Ordnungen der Arthropoden.' 

 Bronn's ' Theirreich,' Bd. v., 1866-79. 



330. BRANDT, A., 'Ueber das Ei und seine Bildungsstatte. Ein ver- 

 gleichend -morphologischer Versuch mit Zugrundelegung des In- 

 secteneies.' Svo, Leipzig, 1878. 



Brandt gives a bibliography with references to 248 papers, and many have 

 been published since. Those which treat of the ova of Arthropods more or less 

 directly are, however, only 90 in number, some very lengthy ; and in wading 

 through this immense mass of literature one cannot but be struck by its 

 barrenness continual repetitions of a few known facts, with lengthy discus- 

 sions as to the nature and origin of the germinal vesicle. All the observations 

 before 1878, including Brandt's, were made by methods which are now 

 obsolete, and deal with the deceptive appearances produced by ' optical 

 sections.' 



331. SCHNEIDER, A., ' Ueber die Geschlechtsorgane der Insecten.' Zoolo- 

 gische Beitrage, Bd. i., 1883. 



332. PALMEN, J. A., 'Ueber paarige Ausfuhrungsgange der Geschlechts- 

 organe bei Insecten.' Svo, Helsingfors, 1884. 



333. LOWNE, B. T., ' On the Structure and Development of the Ovaries 

 and their Appendages in the Blow-fly.' Journ. Linn. Soc. Lond. Zool., 

 vol. xx., 1889. 



334. TETENS, H., 'Resultate der anatomischen Untersuchung eines 

 lateralen Zwitters von Smerinthus Populi, nebst einigen daran ge- 

 kniipften allgemeinen Betrachtungen.' Berlin Entom. Zeitsch., 

 Bd. xxxvi., 1891. 



