336 THE DEVELOPMENT OF THE NYMPH. 



theory.' What this means is beyond my comprehension, but 

 it is clear to me that Van Rees' mesenchyme, in the regions 

 from which the muscles are developed, consists of different 

 elements, both parablastic and mesoderm cells. As to the 

 origin of these cells, his opinions are, in my judgment, incap- 

 able of being maintained, as he has not been able to distinguish 

 the mesenchyme (parablast) from the mesoblast. Neither do I 

 regard his account of the development of the wing muscles as 

 consonant with facts which have been repeatedly observed by 

 myself and others. Whether Ganin really recognised the 

 mesoderm as the inner disc cells, as his statement as to their 

 origin leads me to think, is a question I cannot settle, as I am 

 only acquainted with his work from the translations of Hoyer 

 and others, as already stated. 



h. Development of the Dorsal and Sterno-Dorsal Muscles. 



Van Rees is of opinion that the dorsales and sterno-dorsales 

 of the imago (' Brust Muskeln ') are developed directly from 

 larval muscles. The development of the muscular system of 

 the imago has hitherto presented great difficulties, and the 

 most diverse opinions have been held as to its origin. Weis- 

 mann thought that the muscles are formed from granule cells ; 

 Kiinckel d'Herculais derives them from the discs; Ganin 

 from the disc mesoderm ; Viallanes retroverted to a view 

 similar to Weismann's, and regarded them as developed from 

 cells originating in the fat bodies ; and Kowalevski returns to 

 Ganin's view. In the face of the confusion which has arisen 

 from not defining the exact meaning of mesoderm, it is very 

 difficult to understand how far Ganin's and Kowalevski's views 

 correspond. But it is clear that great differences of opinion 

 exist on this point. 



Van Rees, on the evidence of several sections of the pupa in 

 the first day, which show three pairs of dorsal muscles of the 

 larva in the second thoracic segment, in a less degenerated 

 condition than those around them, concludes that these become 

 the wing muscles (dorsales). Accepting all Van Rees' facts, it 



