1885.1 PROCEEDINGS OF UNITED STATES NATIONAL MtJSEUM. 141 



embryos will be necessary before it is possible to cotuprehend the meau- 

 ing and importance of the vascular digitations at the edges of those fins. 

 All of them receiv^e their blood-supply from the median aortic trunk. 



The trunks given off to the dorsal tin traverse the median plane of 

 the body and tail, and pass up in a nearly straight direction to the base 

 of the dorsal, at irregular intervals of one, two, or even six muscular 

 segments. Upon reaching the base of the tins they subdivide into from 

 two to six branches, which pass up a little to one edge of the interradial 

 space, but give oft" smaller trunks along their entire course through the 

 interradial membrane, which is highly vascular. Continuing they pro- 

 ceed to the margin of the fin. They end in a flat sieve like capillary 

 mesh, which fills up almost the whole of the marginal lobes formed by 

 the prolonged interradial membranes. The tissue of the walls of the 

 vessels within these terminal interradial lobes seems to be mesoblastic, 

 but their external walls next the exterior seem to be mostly epidermic 

 and exceedingly thin, probably not more than three or four cells thick. 

 The capillary mesh is most complex and fiuest in the terminal lobes, 

 less so in the interradial sjiaces. 



The digitations on the spinous portion of the dorsal are very feebly 

 developed, in fact scarcely apparent between the tips of the nine spines 

 of the spinous part of the dorsal of a foetus of Micrometrus aggregatus, a 

 species in which these spines develop, by the way, in a continuous fold, 

 and not in separate pockets or diverticula as in Lophius and Gasteros- 

 teus. * Four principal vessels supply the interradial and terminal capil- 

 lary meshes of the soft dorsal with blood. These four vessels pass in 

 an almost vertical direction from the aorta to the base of the fin, and 

 do not follow the curvatures of the intermuscular septa. No vessels 

 of unusual caliber pass to the spinous part of the dorsal. 



The terminal or marginal interradial lobes of the anal of the foitus of 

 Micrometrus are twenty-six in number, or three in excess of the dorsal, 

 while the two spaces between the anterior spines of the anal are without 

 produced vascular lobes. Five principal vascular trunks pass verti- 

 cally downward to the base of the anal, where they subdivide and sup- 

 ply each of the interradial spaces. The marginal lobes of the anal and 

 dorsal are about alike in length and all have a marginal vessel circum- 

 scribing the capillary mesh at their edges. In this last respect they 

 are like the terminal lobes of the caudal, in which they are, however, 

 somewhat larger and longer. 



From what has been said above it will be noticed by the reader that 

 the number of vascular trunks passing outward to the bases of the 

 vertical fins does not correspond to the number of rajs which they con- 

 tain. This fact indicates that this singular vascular supply of the ver. 

 tical fins of Embiotocoid embrj'os has attained great specialization and 

 must be of very great physiological importance. 



This fact is rendered still more striking when we come to consider 

 the vascular supply of the caudal fin, which is quite unique so far as 1 

 * See Am. Naturalist, 1885, p. 415. 



