248 THE MICROSCOPE AND ITS REVELATIONS. 



virgin or unimpregnated queens, occasionally lay eggs, from which 

 eggs none but drones are ever produced. From careful Microscopic 

 examination of the drone eggs laid even by impregnated queens, Siebold 

 drew the conclusion that they have not received the fertilizing influence 

 of the male fluid, which is communicated to the queen-eggs and worker 

 eggs alone; so that the products of sexual generation are always female, 

 the male being developed from these by a process which is essentially one 

 of gemmation. 1 



644. The Embryonic Development of Insects is a study of peculiar 

 interest, from the fact that it may be considered as divided (at least in 

 such as undergo a ' complete metamorphosis') into two stages that are 

 separated by the whole active life of the larva; that, namely, by which 

 the Larva is produced within the egg, and that by which the Imago of 

 perfect insect is produced within the body of the Pupa. Various circum- 

 stances combine, however, to render the study a very difficult one; so 

 that it is not one to be taken up by the inexperienced Microscopist. 

 The following summary of the process in the common Blow-fly, however, 

 will probably be acceptable. A gastrula with two membranous lamellae 

 ( 391) having been evolved in the first instance, the outer lamella very 

 rapidly shapes itself into the form of the larva, and shows a well-marked 

 segmental division. The alimentary canal, in like-manner, shapes itself 

 from the inner lamella; at first being straight and very capacious, includ- 

 ing the whole yolk; but gradually becoming narrow and tortous, as addi- 

 tional layers of cells are developed between the two primitive lamellae, 

 from which the other internal organs are evolved. When the larva comes 

 forth from the egg, it still contains the remains of the yolk; it soon 

 begins, however, to feed voraciously; and in no long period it grows to 

 many thousand times it original weight, without making any essential 

 progress in development, but simply accumulating material for future 

 use. An adequate store of nutriment (analogous to the ' supplemental 

 yolk ' of Purpura, 584) having thus been laid up within die body of 

 the larva, it resumes (so to speak) its embryonic development, its pas- 

 sage into the pupa state, from which the imago is to come forth, involv- 

 ing a degeneration of all the larval tissues: whilst the tissues and organs 

 of the imago " are re-developed from cells which originate from the dis- 

 integrated parts of the larva, under conditions simiJar ^o those apper- 

 taining to the formation of the embryonic tissues from fche yolk." The 

 development of the segments of the head and body in Insects generally 

 proceeds from the corresponding larval segments; but according to Dr. 

 Weissman, there is a marked exception in the case oj the Diptera and 

 other insects whose larvae are unfurnished with legs v their head and 

 thorax being newly formed from 'imaginal disks' which adhere to the 

 nerves and tracheae of the anterior extremity of the larva; 2 and, strange 

 as this assertion may seem, it has been confirmed by the subsequent in- 

 vestigations of Mr. Lowne. 



645. ARACHNTDA. The general remarks which have been made in 

 regard to Insects, are equally applicable to this Class; which includes, 

 along with the Spiders and Scorpions, the tribe of Acarida, consisting 

 of Mites and Ticks. Many of these are parasitic, and are popularly 



1 See Prof. Siebold's Memoir "On true Parthenogenesis in Moths and Bees," 

 translated by W. S. Dallas : London, 1857. 



2 See his ' Entwickelung der Dipteren,' in "Kolliker and Siebold's Zeitschrif t, " 

 Bande xiv.-xvi.; and Mr. Lowne's "Anatomy of the Blow-fly," pp. 6-9, 113-121. 



