419 



migrated into the degenerate muscle mass, lost their true cell 

 walls, apparently, and growing at the expense of the larval 

 muscles, and probably nourished also by the surrounding 

 blood, formed a great syncytium, on the outside of which the 

 nuclei then arranged themselves. This syncytial mass then 

 broke up into five longitudinal masses, and the further break- 

 ing up of these masses into longitudinal fihrillae led to the 

 formation of structures which, on further differentiation, 

 became the adult muscles of flight. 



These conclusions of Perez are undoubtedly more in 

 harmony with our conception of the nature of cells, and in 

 the main I have been able to verify them; his observations, 

 however, on the differentiation of the great syncytial masses 

 are, I believe, quite incorrect; the process as I have seen it 

 in Nasonia is certainly entirely different. 



Special attention has been drawn to the flying muscles 

 of insects by Schafer (1891), who has formulated a theory of. 

 muscle contraction on the basis of certain structural arrange- 

 ments which he observed in the ''fibrillae" of the flying 

 muscles of insects. The development of the great thoracic 

 muscles of Nasonia shows that in homologieing the "fibrillae" 

 of the flying muscles of the insect with the fibrillae of other 

 muscles. Professor Schafer was incorrect ; (5) at the same time, 

 the false homology will in no way discredit his conception 

 of muscle action. The thoracic muscles of insects, indeed, 

 kre_pe_rfectly ^unique, and deserve to rank, I believe, with the 

 [other types of contractile structures — plain, striated, and 

 [eardiac muscles — as a fourth type of muscle fibre, in which, 

 though striations are present, fibrillae are entirely absent. 

 'The flying muscles of the insect are to be regarded not as 

 .consisting of a great number of fihrillae with remarkably com- 

 Iplex structure, but rather of great numbers of fibres (sarco- 

 styles) in which fibrillae are absent. This will become clearer 

 ^Eelf'we have considered the development of the imaginal 

 bhoracic muscles. 



The great thoracic muscles of Nasonia are in the form of 



ive pairs of longitudinal muscles, lying one above the other, 



[and occupying the greater part of the thorax. Anteriorly 



[they are inserted upon the ingrown extremity of the meso- 



Ithorax, while behind they are attached to more or less 



strongly developed phragmas in the region of the metathorax 



md the propodeum. There are, besides these, five pairs of 



^5) A difference between these fibrillae and those of ordinary 

 "muscles was indeed considered in Schafer's original paper (1891), 

 and they were referred to as sarcostyles. 



L^ 



