00 • NOTES AND MEMORANDA. 



in one of olclcr construction |^. " The only general principle in regard 

 to tlie interval separating the lenses is, that it shall be less than the 

 solar focus of the field-lens ; and when in the deeper eye-pieces and 

 those which are orthoscoiiic it seems to exceed this limit, it must 

 be remembered that in connection with the objective the eye-piece 

 receives diverging rays, and for such its focus is beyond the solar 

 focus. It may also be noticed that but a small part of the diameter of 

 the eye-' ens is actually used in the lower powers." 



Malpighian Vessels of Insects. — Dr. E. Schindler has published 

 an account, with three jdates and a woodcut, of his extended researches 

 on these structures.* This paper gives, first, an general account of 

 the structure of the vessels in question, then an historical summary 

 of the work of former observers, then a special account of the 

 Malpighian vessels in the various groups of insects, and finally some 

 concluding remarks, summarizing the results at which he has arrived. 

 It is only possible here to give some account of the first and last of 

 these sections. 



The Malpighian vessels consist of at least three layers : externally 

 a serous coat of nucleated connective tissue, then a delicate homo- 

 geneous tanica propria, and finally a single layer of glandular 

 epithelial cells bounding the lumen of the tube. To these is some- 

 times added a perforated cuticular tunica intima. Elastic and 

 muscular layers are but little developed, and the flow of the 

 secretion, set free by the dehiscence of the gland-cells, is produced 

 partly by its own gradual accumulation, partly by the movements of 

 the other organs. The tubes may appear white, yellow, brown, green, 

 or red, according to the colour and quantity of their contents. Their 

 size and number vary greatly, their length being, as a rule, inversely 

 proportional to their number. 



The Malpighian vessels are exclusively excretory (renal) organs, 

 and not, as has been supposed, biliary, or both biliary and renal. 

 This is supported by their mode of development as outgrowths of 

 the hind-gut by their early origin, and by the fact that they are 

 functional before any bile is found and while the hind-gut is still a 

 blind jiouch, but chiefly by their close resemblance to the urinary 

 tubules of higher animals, and by the nature of their contents. It is 

 well made out that they contain specific urine-constituents, such as 

 uric acid, acid sodic and amnionic urates, leucin, calcic oxalate, &c., 

 and that no substance not already known in the urine of other 

 animals occurs in them. 



The chief facts tending to support the theory that these tubes are 

 hepatic as well as renal, are the yellow and green colours often 

 observed in them, and the polymorphism of their epithelial cells. 

 With regard to the first of these points, Schindler states that the 

 colour is dependent on a specific colouring matter in the blood 

 plasma, that no bile pigments are present, and that the colour is very 

 inconstant. The polymorphism of the cells was used as an argument 

 for double function by Ley dig, who supposed that certain cells had 

 assigned to them a hepatic, others a renal function. But according 

 * ' ZeitSL'h. f. wiss. Zool.,' vol. xxx. p. 587. 



