xlvii 



spiracle lies between two segments it is " always tlie property of 

 the posterior surface of the segment in front of it." 



Mr. Balfour, in his ' Comparative Embryology,' has given an 

 excellent resume of our knowledge of the Embryology of Insects. 

 He regards Moseley's observations of the tracheal system of 

 Perijyatus as fatal to the view that insects can have sprung 

 directly from aquatic ancestors not provided with trachea, and 

 considers that these characteristic organs are modifications of 

 cutaneous glands. The problematical " imaginal disks" of 

 Weismann he regards as derivatives of the embryonic epiblast, 

 like the similar invaginations of the skin in many larval 

 Nemertines. He considers that the Thysanura and Collembola, 

 while " not belonging to the true stock of the ancestors of 

 insects," are " degenerated descendants of this stock ; much as 

 Amjiliioxus and the Ascidians are degenerate relatives of the 

 ancestral stock of Vertebrates." He considers that the closed 

 tracheal system of larvae with tracheal gills is undoubtedly of a 

 secondary character, and that this tells strongly against the 

 view that the wings are modified tracheal gills. 



We are also indebted to Mr. Balfour for a short but interesting 

 paper on the Embryology of Spiders.* The species which he 

 studied is Agelena labyrinthica. His observations have satisfied 

 him that in their development the Arachnida are much more 

 closely related to the other Tracheata than to the Crustacea ; 

 but I must confess that I doubt whether we have yet sufficient 

 data to justify us in concluding that the points of difference on 

 which he dwells will be found to hold good generally. There 

 has been much difference of opinion as to the homology of the 

 Chelicercs, and some eminent authorities have regarded them as 

 corresponding to the antennae of insects. Mr. Balfour, however, 

 considers that his observations prove them to be postoral, and 

 equivalent to either the mandibles or the first maxillae of other 

 Tracheata. As regards the yolk-cells, he finds himself in close 

 harmony with Dohrn, Bobretsky and Graber ; but he states that 

 the first formed mesoblastic plate does not give rise to the whole of 

 the mesoblast, but that during the whole embryonic life the meso- 

 blast continues to receive accessions from the cells of the yolk. 



M. Fabre has published a charming volume of ' Souvenirs 

 Entomologiques.' He has continued and added to the very 

 * ' Quarterly Journal of Microscopical Science,' 1880, p. 167. 



