174 ME. K. J. TILLYARD ON THE RECTAL BREATHING-APPARATUS 



tLe original nuclei of the epithelial syncytium, which have become enlarged 

 in a given direction by the gradual growth o£ the pad perpendicular to the 

 epithelial plane. 



The meganuclei are chromatophil, staining fairly deeply with hsematoxylin 

 in contrast to the surrounding substance of the pad. Each meganucleus 

 contains a very definite and darkly staining nucleolus, together with numerous 

 scattered granules, also very darkly staining. 



Here and there in the sections of the pad, but more especially towards its 

 ends, there are to be seen smaller nuclei (^nK^), which I propose to distinguish 

 by the name microniidei of the hasal pad. These are nearly always found to 

 lie near the interual border of the pad. They are darkly stained, usually 

 showing a nucleolus and close granular contents. In shape they appear to 

 be flattened or elongated a little in the opposite direction to the axis of rhe 

 meganuclei ; very often they are nearly circular in cross-section. These I 

 also regard as originally nuclei of the epithelial syncytium, which, for some 

 reason or other, have failed to swell out into meganuclei during the develop- 

 ment of the pad. The fact that they lie near the ends of the pad partly 

 explains this occurrence. That they also lie close to the internal border o£ 

 the pad seems to me a strong argument for the development of the pad by 

 imbibition of watery fluid from the rectum ; for, if such be the case, those 

 nuclei which lie closest to the external surface would naturally respond first 

 to the access of fluid. 



In putting forward the above theory as to the derivation of the basal pad 

 and its nuclei, I may add that, in young larvee of Anax and y^scJina which 

 I have examined and sectioned from the first to the fourth instars^ there are, 

 no traces of basal pads. Their development, then — in these genera, at least, — 

 takes place in later stages. This fact seems to me to make the supposition 

 of their development by imbibition of water from the rectum a very probable 

 one — at any rate, it could not be urged ;is a solution if we had found the 

 basal pads already developed in the newly-hatched larva. 



The basal pads of Hemigomplius and Petalura are essentially similar in 

 structure to those of Austrogomplius . Those of Petalura were, however, not 

 so wide in comparison to their length as in the two Gomphine genera. 

 Little weight, however, can be attached to this circumstance, since the larva 

 of Petalura which I sectioned had been in alcohol for over four years, and 

 there were very obvious signs of the complete extraction of the watery fluid 

 from the pads, which in some parts had actually collapsed or broken open,, 

 showing clear unstained spaces between the fibrils. The complete absence of 

 any substance between the fibrils in the pads of this larva points to the 

 probability of their original contents having been largely of a watery nature, 

 and gives further support to the theory outlined above. 



In the three genera under discussion, as well as in the u^schnhue, I have 

 never seen, on opening the gill-basket, any sign of definite basal pads of a 



