DORSAL VESSEL OF THE IMAGO OF THE BLOW-FLY. 647 
According to Graber, the alar muscles in most large insects 
—Orthoptera, Coleoptera, Hymenoptera and Lepidoptera— 
are united with the heart itself, so that it is evident that they 
cannot be separated from it by a continuous fenestrated layer, 
as they are inthe Blow-fly. In these insects the muscle-fibres 
appear to lie in the substance of the fenestrated layer, and 
probably perforate it obliquely. 
Graber, who has examined the structure and chemical rela- 
tions of this merabrane with great care, says it is not exactly 
like any of the tissues of Vertebrates, but resembles most 
closely the ligamentum pectinatum iridis. I regard it as a 
cuticular membrane derived from the connective-tissue cells 
of the septum, so that it is either continuous with the myo- 
lemma of the muscle-fibres, or covers both their surfaces in the 
median region of the septum. Graber also states that it is 
sometimes continuous with the tunica adventitia of the heart. 
So that it may be reflected over the dorsal vessel as a thin 
subendothelial layer in some cases. 
The cells which are adherent to the pericardial septum are 
termed by Graber ‘pericardial’ cells. 
Pericardial Cells.—I think it is evident from both the figures 
and descriptions given by Graber, that he includes very dif- 
ferent forms of cells under the term ‘ pericardial.’ He says: 
‘In conjunction with R. Wagner, Leydig speaks of a special 
cellular layer of the heart, which consists of large cells with 
green or brown pigmented contents, and of a continuous trans- 
parent ground or connective substance, in which the tendons 
of the alar muscles are spread out and lost; and Weismann 
further states that the cellular layer is connected with the 
heart by a loose muscular sheath in which the alar muscles are 
imbedded.’ Graber adds: ‘ My own observations lead me to 
a view not consonant with the above, for the pericardial cells 
form a single peculiar layer enclosing the space in which the 
heart lies, and not infrequently have no connection either with 
the heart itself or the alar muscles, but are continuous with the 
connective layer of the hypodermis.’ 
As there is no connective layer of the hypodermis, I suppose 
