656 CIRCULATION OF THE BLOOD AND DORSAL VESSEL 



vessel frequently recommence. I attribute this to the removal 

 of the inhibitory influence of the ganglion cells attached to the 

 pericardial septum. 



The blood which enters the pericardial septum passes over 

 the great air-sacs of the thorax and abdomen, and from this 

 fact, and the large number of fine tracheae on the septum, there 

 can be little doubt it is charged with oxygen. The dorsal 

 vessel is, therefore, a true arterial heart, like that of the 

 Crustacea, which sends arterial, i.e., oxidized blood to the 

 head. 



4. THE DEVELOPMENT OF THE DORSAL VESSEL. 



The manner in which the dorsal vessel is developed in the 

 embryo Insect is insufficiently known. Weismann says [2, p. 85], 

 describing the development of the embryo of the Blow-fly : 

 ' From the position of the dorsal vessel in the larva, it is 

 possible to conclude that it is developed from the deepest of 

 the superficial cell layers of the embryo. During embryonic life 

 I have not been able to observe it, although it is beautifully 

 seen in the posterior part of the body of the young larva, in 

 which it contracts 56 times in a minute.' 



Biitschli [126] states that in the Bee embryo it is developed 

 from two cell strings, which form the edges of the somatopleure 

 from the neck to the posterior extremity of the larva ; but he 

 gives no details, and docs not indicate whether there are 

 several rows of cells, or only one row on each side. Jawo- 

 rowski [315] admits that he was unsuccessful in tracing the 

 development of the dorsal vessel in the embryo of Chironomus, 

 and I have been equally unsuccessful in the I How-fly embryo. 



Bibliography : 



314. DAK i.- 1 i , < '., 'Note sur !e Dcveloppcment clu Yaisscau dorsal chez 

 les Insectos.' Archiv. cl. Xool. experimental, torn, ii., 1873. 



315. |A\VOII\\>K.I, A., ' Ueber die Kntwickelung des Riickengefasses und 

 speciell der Musculature bei Chironomus und eini^en andcren 

 Insecten.' Sit/.b. der k. Acacl. dcr \Vissensch. \Y~ien., Svo, 15d. Ixxx., 

 ' 79- 



