1885.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 161 



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Vol. Till, TVo. 11. Washington, D. €. June 1, 1885. 



si, Fig. 3. This vessel passes forward under and iuto the liver L, where 

 it agaiu breaks up into a plexus of smaller vessels; these then again 

 blend iuto larger trunks, as shown at j)v, in Fig. 4, which emerge from 

 the liver to again break up into the capillary net-work cc, Fig. 3, which 

 is still better shown in Fig. 1, on the surface of the yelk-bag. This 

 vitelliue network then joins the vitelline veins w and vv\ which blend 

 into a common trunk before joining the venous end of the heart. It 

 will thus be seen that we have no less than five capillary systems in 

 the young salmon, as shown in Fig. 1, if we reckon those of the portal 

 system together with those belonging to the systemic system of vessels. 

 These in their order are: (1) The branchial, (2) the systemic, (3) the in- 

 testinal, (4) the hepatic, and (5) the vitelline capillaries. 



What the subsequent history of the third and fourth sets may be I 

 have not made out, but the fifth or vitelline set has only a temporary 

 existence, remaining only as long as there is yelk in the yelk-bag. As 

 the yelk is absorbed this system disappears, when the vitelline veins vv 

 and vv' become portal veins; that is, they carry all of the blood which 

 passes through the viscera back to the heart. 



A study of sections of the yelk-sac of the salmon leads to the follow- 

 ing conclusions: A well-marked periblastic stratum of plasma, |j, Fig. 

 4, invests the yelk. Beneath the periblast lie the oil-drops o, o, which 

 are largest at the upper part of the yelk, the greater buoyancy of these 

 larger, superior oil globules tends to keep the young fish buoyed up, 

 and functions much in the same way that an air bladder would, a struct- 

 ure which, by the way, is not yet functional in the young salmon at 

 this stage of growth. In the earlier stages these larger oil-drops, which 

 lie just under the blastodisk or germinal mass, by their buoyancy con- 

 stantly keep the germ rotated or turned toward the top of the ^gg. 



External to the periblastic layer of the yelk comes the vascular net- 

 work of capillaries, the walls of which are formed, apparently, by a 

 thin sheet of splanchnic laesoblast vm, which invests the yelk but 

 which has grown down over the latter at a later period, possibly, than 

 the thin epidermic or epiblastic investment ep. Fig. 4. This vascular 

 net- work is obviously the apparatus by means of which the yelk is ab- 

 sorbed superficially from the external Plasmodium or periblast of the 

 yelk, a stratum, which, as is well known, contains scattered free nuclei. 



The epidermis or epiblast of the young salmon is remarkable amongst 

 fish embryos for the peculiar goblet-shaped cells which are found dis- 

 tributed over almost the entire surface of the embryo. These are shown 

 in a section of the epidermis in Fig. 5, much enlarged. Their function 

 is apparently to secrete a mucilaginous substance for the purpose of 

 protecting the skin of the embryo. 



IV. Diseases or ahnormalities which involve the yelk-sac and vitelline 

 Proc. Nat. Mus. 85 11 



