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have had upon it is lost, unless the effect is preserved in the seed ; and it does not matter how many generations have lived under the given uniform environment, for the plant starts all over again, de novo, each year. Therefore, the environment must affect the annual plant in some one generation or not at all. It seems to me to be mere sophistry to say that in plants which start anew from seeds each year, the effect of environment is not felt until after a lapse of several generations, for if that were so the plant would simply take up life at the same place every year. This philosophy is equivalent to saying that characters which are acquired in any one generation are not hereditary until they have been transmitted at least once!
My contention then, is this: plants may start equal, either from seeds or asexual parts, but may end unequal; these inequalities or unlikenesses are largely the direct result of the conditions in which the plants grow; these unlikenesses may be transmitted either by seeds or buds. Or, to take a shorter phrase, congenital variations in plants may have received their initial impulse either in the preceding generation or in the sexual compact from which the plants sprung. Cornell University, Ithaca, N. Y.
A COMPARATIVE STUDY OF THE POINT OF ACUTE
VISION IN THE VERTEBRATES.
BY J. R. SLONAKER,
Fellow in Biology, Clark University. In this preliminary sketch of a comparative study of the eyes of vertebrates, with special reference to the fovea centralis or point of acute vision, I shall first give the processes and methods of preparation which I have used and results obtained, and, second, the position of the area centralis as indicated by the retinal arteries. The microscopic descriptions and the relation of the position and shape of the eye and arrangement of the retinal elements to the habits of the animal will follow in a later paper.
"I wish to thank Dr. C. F. Hodge for valuable assistance and for his method of injecting the eye-ball, thus preserving it for complete sections. I am also very much indebted to Clark University for valuable aid and for apparatus and materials to further this study.
For microscopical purposes and best results it is necessary to obtain the eye fresh, at least not later than an hour after death, and subject it to the action of certain hardening liquids which will permeate and preserve without causing the retina to swell and become wrinkled. With some animals it is quite easy to preserve the retina without its becoming wrinkled or floated off (fishes, amphibians, reptiles, and some mammals), while with others (most mammals and birds) it is a more difficult task.
In order to prevent this folding and floating off of the retina, the eye is injected under pressure and immersed at the same time in a bath of hardening fluid. It is carried thus on up through the different percentages of alcohol and imbedded in celloidin.
A more minute descriptior of the method is as follows: Fig. 1 represents a rack with movable shelves, on which are placed bottles A and A', containing the same fluid as bottles B and B', and provided with siphons to connect with glass cannulas.
In order to insert the A
cannula, a hole is carefully drilled about the equator and on a meridian perpendicular to the plane in which it is desirable to obtain sec
tions. The perforation B'
is stretched open, rather B
than cut, so the sclerotic will clasp the neck of the cannula tightly. A convenient instru
ment for this operation FIG. 1.
is a spear-pointed dissecting needle, and not
too sharp. At the same time reach forward with the point of the needle and pierce the suspensory ligament and iris in order to open the aqueous chamber. In doing this, care is taken not to injure structures in the plane of the desired sections. A cannula of suitable size, being connected with a siphon from A or A', is filled with the liquid and inserted. The cannula should have a fine smooth point. Great care is taken in inserting it so that the stream of fluid is not directed behind the retina to float it off. A hole is now made in the opposite side of the eye, the aqueous chamber again pierced and all aqueous and vitreous humor allowed to run out. In some animals this huinor is very much more gelatinous than in others, and requires much more pressure to remove it! The hole below is then stopped with a small glass plug (Fig. 2, B), and the eye immersed in hardening fluid (Fig. 1, B). The bottles are now covered as tightly as possible with tinfoil to prevent evaporation and entrance of dust particles. The cannula and stopper should fit so tight that there is no leak. In every case the orientation of the eye is marked before it is removed from the
FIG. 2. head. This is done by sewing a small tag to the outer layers of the sclerotic (Fig. 2, C).
The pressure varies greatly with the kind of eye used. Those with thin walls, or containing much cartilage, birds and amphibians, require little pressure, while mammals, in general, can receive much higher. The pressures which I have found to work best vary between 28 and 36 cm.
The hardening fluid used is Perenyi's, in which the eye is allowed to remain twenty-four hours, when it is changed to 70 per cent. alcohol.
In making changes of liquids, great care should be taken that no air get into the eye, and that all the former liquid is
tory outal see
replaced with fresh by removing the stopper in the lower part of the eye. After remaining twenty-four hours in each of the following liquids : 80, 90, 95 per cent., absolute alcohol and absolute ether (1 part each), it is then changed to celloidin. Best results are obtained when three grades of celloidin are used—1st, very dilute; 2d, less dilute; 3d, as thick as will run. It is allowed to remain from four to six days in the first, six to eight days in the second, and ten to fifteen days in the third. If the eye is kept well under pressure throughout this process, the retina will be well preserved and lie smoothly against the choroid.
I have tried other liquids for hardening the eye whole, but with poor success. Have tried the method of Barrett and of Cuccati, but, in each case, the retina was very much wrinkled and folded, while the whole eye was much shrunken and out of shape. In vapors of osmium, I have had fairly good results with the retina, but the same trouble, due to the shrinking of the whole eye, is present. Chievitz says that a fish's eye may be preserved whole, with retina lying nicely back, by simply immersing it, or even the whole head, in 80 per cent. alcohol. The hardening agent which he generally uses is 2.5 per cent. nitric acid.
Another method which I have employed with small animals, especially birds, in order to demonstrate quickly the presence or absence of a fovea, is to immerse the whole head in Perenyi's fluid for from three to five hours. This will harden the eyes so that the cornea, lens and vitreous humor may be removed, leaving the posterior half in situ. With birds I have had good results, the retina lying back smoothly so that the fovea and entrance of the nerve, marked by the pecten, may be easily seen. Fig. 3 represents diagrammatically the appearance of the retina after the front of the eye has been removed.
In order to show the angles which the lines of vision make with the median plane, sections were made through the whole head of several animals (fish, amphibians, reptiles, birds and
? J. H. Chievitz, Untersuchungen über die Area centralis retinae. Archiv für Anatomie und Entwickelungsgeschichte, Sup., Band, 1889, p. 141-142.
small mammals), the plane of the section passing through each fovea on the centre of the area centralis. Fig. 4 represents such a section through the foveæ a and b of a chickadee's head (Parus atricapillus), while the lines G H and GI show
A and B. Fovea.
the axis of vision. The dotted lines c mark the position of the optic nerves which enter in a plane much lower down. In order to harden the whole head, and, at the same time, decalcify the bone, it must remain longer in Perenyi's fluid (about thirty-six hours), and to preserve the cornea and lens in position, a window is made in the top of the eye that the fluids
Having had good success with simple immersion of the head, this method was tried for hardening the small eyes, and with good success. In fact, the retina proved in good condition, if not better, than when taken through by the injection method. The eye-ball, however, usually caves in when placed in 70 per cent. or 80 per cent. alcohol, but this may be prevented by simply making a small slit through the sclerotic