256 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
pies, in part at least, exactly the position of the ligamentum longitudinale which tra- 
verses longitudinally the neural arches of the other groups, embracing the true Ashes. 
The Abrous cords traversing this organ, as seen in the lampreys, may, through 
degeneration of the lymphoid tissue, have given rise to the ligamentum longitudinale, 
through some common ancestral type. 
This lymphoid organ in the lamprey also contains black pigment cells mixed 
amongst the lymphatic tissue, but the organ extends for the greater part of the length 
of the vertebral column in the lampreys. 
In the sturgeons there is no cord of lymphoid tissue extending above the spinal 
cord, for its whole length, but at its anterior end the spinal canal is greatly widened, 
and in this dilated portion of the latter there is a mass of lymphatic tissue included 
which is not traversed, as in the lampreys, by Abrous connective tissue, but as in 
them is found to contain scattered black pigment cells. This mass of lymphoid tissue 
in the sturgeon lies partly within the dilated anterior part of the spinal canal and 
partly within the skull, and extends down over the sides of the anterior end of the 
spinal cord and medulla oblongata. This mass of lymphatic tissue therefore occupies 
partly an intracranial position and does in fact extend slightly in front of the auditory 
region of the skull. 
While it is diflAcult to identify the lymphoid tissue found in such close relation 
with the nervous system of the sturgeon with the massive trihedral lymphoid strand 
overlying the spinal cord in the lamprey, the comparison is at least suggestive. 
A still more remarkable lymphoid organ is found investing the ventricle of the 
heart and the bulbus aortm of the sturgeon. It is one of the most striking structures 
found in the sturgeon, and at once attracts the attention of the anatomical tyro upon 
opening the cavity in which the heart lies. It is, in fact, the tissue which gives to 
the surface of the heart of the adult a lobulated appearance entirely different from that 
of the heart of all other Ashes. These lobules are the outward expression of comjiart- 
meuts in a thick layer of lymphoid tissue, which are found to be lined with jirocesses 
that are produced into plates, or lobes, internally that are often dentritic or branched 
as seen in cross-section. In portions of this structure lymphatic tissue has been found 
of the usual type met with in higher types, but no muscular Abers have been observed. 
Besides this there have been found masses of a homogeneous substance in some of the 
lobules, which recall the masses of similar matter seen in the closed sacks of the 
thyroid body throughout the vertebrates geuei'ally. Its function has never been 
clearly determined, but my own opinion is that it probably belongs to the category of 
ductless glands and tliat it may have some relation to the thymus or thyroid, probably 
the latter, as long ago suggested by J. F. Meckel. The history of this singular organ 
can only be made out deAuitely by a study of its development, from fresh materials, 
extending over a great many more stages than are at my command for that purpose. 
Until that is done it will be useless to speculate as to the true nature of the singular 
lymphoid structure which covers nearly the whole heart of the sturgeon. It com- 
pletely covers the ventricle and bulbus; is Armly attached to their outer surfaces, and 
forms, so to speak, a second, and highly differentiated wall lying external to the mus- 
cular coat of the heart. Only one other suggestion occurs to me as respects its true 
nature, and that is that it may possibly be a greatly thickened epicardium in which 
lymphatic tissue has been developed, but even this suggestion would have to be veri- 
Aed by further embryological research, for which the required material is not yet at 
hand. 
