THE DEVELOPMENT OF THE NYMPH. 341 



Kowalevski [145] states that in the Blow-fly pupa it is seen 

 pulsating on the third day as it does in the larva, but that 

 later its pulsations are irregular, and he observed that the 

 anterior and middle portions of the dorsal vessel remain 

 almost unchanged during the whole pupa period. 



Weismann, on the other hand, concluded that in the Muscidfe 

 the dorsal vessel degenerates and is rebuilt in the pupa. He 

 says: 'Although direct observations on the cessation of its 

 pulsations have not been made, it may be concluded from its 

 changes of histological structure that after a certain time con- 

 tractions are no longer possible.' 



' In the first days of the pupa state the dorsal vessel remains 

 unaltered. The histolysis of the great tracheae of the larva, to 

 which its alar muscles are attached, removes its point d'appui. 

 Its position is still, however, in the middle line of the back, 

 although it is probably much folded by the shortening of the 

 body of the larva. The ring (see p. 78), also degenerates on 

 the fourth or fifth day, and removes its anterior attachment. 

 The isolation of the vessel at this period is exceedingly diffi- 

 cult, as it has become very fragile, and is evidently in the 

 first stage of histolysis. As an organ, it is not broken up, but is 

 redeveloped by a similar process to that which has been 

 observed in the intestine and Malpighian vessels.' 



The main point to which I would draw attention in Weis- 

 mann's statement I have printed in italics. The rest is not 

 entirely accurate. The ring is not lost as a point of attach- 

 ment, although it is profoundly altered. As an epithelial struc- 

 ture it no longer exists, but a vast number of cells appear 

 around it before it disappears, and these cells form a dense 

 network which remains apparent in the pro-imago, and is a 

 conspicuous object in sections on the tenth day of the pupa, or 

 even later. 



This network of cells lies in the nymph above the hemi- 

 spheres, and subsequently behind them, and appears to travel 

 back over their surface as the head is developed. 



The muscles which form the alas of the pericardium are, it is 

 true, removed, and a pericardium can no longer be said to 



