BULLETIN OF THE UNITED STATES FISH COMMISSION. 159 



ing esseiitiiilly a portal system of vessels. The blood is forced tbrougli 

 the braucliial arches, aud passes from them into the carotid arteries aud 

 aorta, to return to the heart again by way of the jugular and cardinal 

 veins aud visceral network. The venous or dorsal end of the heart is 

 divided by the intestine, the Cuvierian ducts opening on either side of it 

 upwards and backwards. 



It will be worth while to here notice the fact that in the mackerel 

 there is nothing which is comparable with a system of vitelline vessels, 

 such as is found in tlie young salmon, stickleback and silver gar, but 

 that the venous end of the heart is throughout the whole of embryonic 

 life closely applied to the surface of the yelk sack (see Figs. 14, 15, 16, 

 and 17), so that it appears at times almost like a parasite sucking at 

 the vitellus. Stray colorless blood corpuscles may sometimes be seen 

 in the pericardiac cavity, say about a day after hatching, which would 

 indicate that the genesis of the blood from the vitellus was essentially 

 the same as that in the silver gar, where the fluid surrounding the heart 

 contains multitudes of colored blood disks, and where one can observe 

 them in every stage of metamorphosis from the substance of the vitel- 

 lus, at the point where the long tubular venous sinus of these*embryos ' 

 joins the hypoblast of the yelk. At the end of the fourth day the 

 month of the young mackerel is wide open and the lower jaw, in conse- 

 quence of the greater length of Meckel's cartilage, reaches nearly as far 

 forward as the upper jaw or snout. The mouth is also frequently opened 

 and closed at this time by the mandibular and hyoid muscles. The 

 chondrocranium has also advanced in development in all of its parts, 

 since the auditory capsule is now clearly inclosed in an investment of 

 cartilage cells, which are joined to the notochord anteriorly by the de- 

 velopment of the so-called parachordal cartilages. Under the brain, 

 the primitive cartilaginous bars, the trabecule cranii, are developed, 

 and an oval space exists between them, into which the hypophysis cere- 

 bri or pituitary body dips downwards. On either side, and below the 

 hinder half of the eyes, the pterygoid cartilages have been developed 

 at the same time hyoid, quadrate, hyomandibular, ceratohyal, branchial, 

 and coraco-scapular elements have advanced in development. 



The method which I have found the best to demonstrate the carti- 

 lages of the head in fishes as small as the mackerel, which measm^es 

 only a little over one-seventh of an inch at this stage, is the following: 

 crush a recently killed specimen under a Fol's compressor so as to flat- 

 ten it sufiiciently to let the light through it, then use a one per cent, so- 

 lution of acetic acid so as to bring out the contonrs of the cartilages 

 with their cells and nuclei. If this is carefully done there will not be 

 suflicient displacement or disarrangement of the parts of the skull to 

 render their identification at all difficult. 



Figure 17 represents the head of a young mackerel nearly a week old 

 dissected so as to show the structure of the skull as nearly as such a 

 diflticult subject admits of representation. The parts of the chondo 



