THE I M AGIN A L DISCS. 79 



nerve-centres, and passes between the hemispheres ; immedi- 

 ately in front of these there is a ring of cellular tissue, with a 

 lumen large enough to transmit the vessel. The ring hangs 

 freely in the body cavity, and is fixed by fine tracheal vessels. 



' In the anterior part of the second segment ' (my fourth 

 somite) ' a tracheal branch arises from the main trunk, passes 

 inwards and backwards, and ultimately penetrates the hemi- 

 sphere. In its course this trachea is united to its fellow by a 

 transverse vessel which lies in the upper ' (anterior) ' part of 

 the ring; whilst the trachea courses through the side and 

 back part of the ring. The peritoneal coat of these vessels is 

 fused with the tissue of the rmg, so that the latter might per- 

 haps be regarded as a development of the peritoneal layer of 

 the tracheae. This is, however, not so ; the tracheae have little 

 to do with the formation of the ring, as this is clearly an organ 

 which originates in the embryo ; its form is that of a simple 

 broad finger-ring, the upper segment somewhat notched in 

 the middle line. The diameter of the ring is about '23 mm., 

 measured from before backwards ; the dorsal vessel is attached 

 to it, dilates in front like a funnel, and is finally attached to 

 the pharynx. 



' There can be no doubt the ring is a skeletal structure, and 

 in this relation I have not fully described it, for it gives off an 

 anterior and a posterior band.' At this point Weismann's 

 description becomes so complex that I shall only give a resiuiic 

 of his meaning as I understand it. The anterior band con- 

 nects the anterior (upper) part of the ring with the cornua of 

 the cephalo-pharynx — this is my cephalo-pharyngeal band ; the 

 posterior band is continued over the oesophagus and termin- 

 ates in a transverse enlargement in front of the proventriculus, 

 to which it is firmly attached. This posterior band is un- 

 doubtedly the stomogastric nerve, which is connected through 

 the frontal ganglion with the crura of the hemispheres and the 

 dorsal vessel by numerous nerve-fibres, which lie upon the sur- 

 face of the ring, and which were overlooked by Weismann. 



I have not the slightest doubt the cephalo-pharyngeal 

 band is the remains of the invagination of the procephalic 



