160 PROCEEDINGS OF UNlTEt) STATES ISfATiONAL MUSEIjM. [1885. 



the pelvic fin not displaced forward, but normal in position, as in the 

 majority of existing Physostomous Actinopteri. 



The belated development of the pelvic fins of some of the Isospondyli 

 and Haplomi is quite remarkable; in fact, the pectoral in these forms 

 may liave the permanent rays pretty well developed before the ventral 

 fin-fold has done much more than begun to develop. The ventral tiu- 

 fold, however, in those forms in which it appears synchronously with the 

 pectoral is at first always the smallest, thus showing the effects of an 

 inherited tendency to retard the development of the pelvic fins. 



In the salmon the pectoral is functionally developed with well de- 

 fined actinotrichia lying beneath the investing epidermis, as shown in 

 Pig. 1, by the time the ventral is appearing as a flat, immobile lobe 

 without developed actinotrichia, behind and above the yelk-bag. 



III. The embryonic blood vascular system. — In the embryo salmon this 

 attains great importance apparently in consequence of the presence of 

 a voluminous yelk which is absorbed by a system of vitelline vessels. A 

 large median aortic trunk ao, Pig. 3, is developed under the notochord 

 c/t, Pig. 4. It is formed from two convergent suprabranchial arteries an- 

 teriorly, which receive their blood from the branchial trunks coming 

 from the gills. The aorta extends backward, giving off "intercostals " 

 and intersegmental branches s along either side and terminates under 

 and at the end of the urochord. A recurrent vessel then bends down 

 from it and divides into several loops which converge in the caudal 

 venous sinus sc. Pig. 3, incorrectly referred to as a " caudal heart" by 

 some writers, as it exhibits no independent pulsations of its own. 



Prom the caudal sinus the caudal vein or cava arises and passes for- 

 ward towards the body-cavity, where it divides anteriorly into the 

 paired cardinal veins cv, Pig. 3. These pass forward to the venous sinus 

 of the heart formed by the Cuvierian ducts into which they empty their 

 contents. The heart lies behind and below the branchial frame-work, 

 and forces only venous blood through the gills. Its anterior chamber or 

 bulbus aortte is prolonged forward into a truncus arteriosus, which gives 

 off" a pair of vessels to each pair of gills. These vessels, after break- 

 ing up into a plexus of capillaries, send the arterialized blood through 

 another set of branchial vessels which join a pair of longitudinal trunks, 

 of which the radices aort.e are prolongations posteriorly and the caro- 

 tids anteriorly. The venous blood from the head is carried back to the 

 heart by way of a pair of jugular veins or anterior cardinals to the 

 venous sinus. The foregoing describes in outline the systemic circu- 

 lation proper, exclusive of the great portal system, to which the net- 

 work covering the yelk-bag also properly belongs. 



The portal system of the young salmon with the yelk-bag still at- 

 tached may be said to comprise no less than three successive sets of 

 oapiliaric'S. The first of these arise from the caudal and cardinal veins 

 cr, and pass from above downward on either side of the intestine to 

 join and fill with blood a large azygous or median subintestinal vein, 



