22 m ^—m. m « m ^. mm :^ü m m 



place secreted, ^;o tliat it satisfactorily undergoes therein the transformation into the 

 sericin. In this way we find, in the posterior division, the sericin layer cos'ering 

 the fibroin mass. 



6) As to the motive of shifting forwards the silken column in the lunuMi of 

 the gland, there is no inttlligible view advanced by previous authors. Som;' ;uith<>rs 

 ascribe this partly to the action of so-called fliere in the sjiinneret and partlv to 

 the blood pressure. The silken column in the gland-lumen, however, is not a solid 

 body, but nothing more than a viscous fluid ; then, tlic first half of this view 

 self-evidently loses its power ; the second half is unintelligiljle, because the pressure 

 of blood, which slowly flows hachoards in the so-called bt^dy cavity of caterpillar, 

 may not act as motive pressing fonmr(h the secretes. In i!i\- opinion, the motive 

 force in cpie.stion is quite different. Numerous air-bubbles are seen in the interior 

 of the silken column, and a large air-reservoir is found between th(> inner wall of 

 the secretory cells and the central silken cohunn, a fiicl which is so striking that 

 it may not be overlookd at any rate. This enormous iimsxint of air is doubtless 

 brought in by tracheal passages which give it off within ihe cell-bodies from their 

 free terminations; therein the air is driven into the gland-lumen, where it becomes 

 accumulated to preserve a considcj-able pressure. The pu.-ttrior end of the gland 

 being totally blind, this ]n-e.ssure acts on the silken column, so tiiat this fitrce is 

 utilized to shilt forwards the latter. 



7) The anterior division is not excluded from the silk-pi'oiluction, but .secretes, 

 in the embryonal stages, some silken fluid, us in the other two divisions, although 

 the silk secretion is entirely given oif, wIk n the embryo Iialches out, to be followed 

 by secretion of the chitinous .^ubstancc^ wln'ch mak<'s iij> the exceedingly thick 

 intima. 



8) During the first age, the cell-nuclei are nearly Ui!il()rm in shape anil .size 

 throughout the whole extent of the silk gland : they vajy from roundi.sh to ellipsoid. 

 Fuikation of nuclei appears for the first time early in the s 'cond age in the anterior 

 division, being not coincident with the observation by Hklm. ^ 



9) In later stages of the larval life, the nuclei of the anterior division distinctly 

 dificr from tho.se of tlu other divisions in microscopical features : in the firmier 

 part th.ey are slender, homogeneous, non-granulated and less ramified, while in the 

 latter part they are ma.ssive, roughly granulated and complicatedly branched. 



1) F. E. IK'lin: Uvher die Spinndriison dor Jx'pidoptorcn. Zcitxclir. f. (r/.s.s. Zocl. XXVI. ISTG. 

 pp 4?:-[ß9. 



