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PROGRESS OF MICROSCOPICAL SCIENCE. 
scattered, and with the intervals filled with small circular disks. 
In other instances large circular or oval disks occupy the fundus 
of the test, and small ones extend from one-half of the body 
to the mouth, sometimes mingled with a few of the larger disks. 
In some instances the test is composed of minute circular disks, 
alone, or with a few large oval or larger circular ones scattered 
here and there. Generally the disks of the tests are sharply 
defined, closely placed, and touching at their contiguous edges. 
Sometimes they are crowded, and assume in a certain focus a more or 
less polygonal outline. Sometimes they appear to overlap the edges. 
Usually very distinct; they are sometimes more or less indistinct. 
The large disks in a certain focus appear centrally shaded, and 
exhibit a striking resemblance to ordinary blood-corpuscles. Not 
unfrequently the test is mainly or almost entirely composed of minute 
rods, placed in alternating oblique patches, with a few minute round 
disks. In other tests the disks predominate. In some tests large 
and small disks and rods are intermingled. In other tests larger, and 
fusiform rods, probably diatoms, are mingled with disks. Between 
the structural forms of the tests indicated, all sorts of intermediate 
forms are found. Occasionally, mingled with the more intrinsic 
elements of the tests, there are undoubted diatom cases, and rarely 
distinct and comparatively larger particles of siliceous sand. Pro- 
fessor Leidy looked upon the disks and rods of the tests of Nebela as 
intrinsic structural elements. They appear to be siliceous, as they 
undergo no change in heated sulphuric acid. No similar elements 
could be detected among the ordinary materials among which the 
animals lived. Dr. Wallich regards the disks and rods, of the forms 
he has called Difflugia pyriformis var. symmetrica, as being derived 
through the metamorphosis of diatom cases, through the combination 
of these with the basal substance of the test. In the reference to his 
figures 27 to 33, ‘ An. and Mag. Nat. Hist.,’ 1864, he says that 
they “ represent the series of forms exhibiting the transition from the 
ordinary mineral and chitinoid elements of the test to the evolution 
of the colloid disks.” Professor Leidy remarked that notwithstanding 
he had examined multitudes of Nebela, he was not prepared to confirm 
this view, though he had too much respect for Dr. Wallich’s accuracy 
of observation to doubt its correctness. The author then describes 
the various forms, and refers to adjoining figures of them. 
Development of the Nerves in the Chick. — Mr. A. Milnes Marshall 
has communicated an important paper to the ‘ Proceedings of the 
Boyal Society ’ (No. 17 9) on the development of the animal nerves. He 
says that in the investigation he undertook, embryos of ages from 
thirty-six hours to four days were employed. These were, for the 
most part, hardened by immersion in picric acid, prepared after 
Kleinenberg’s method, for three to five hours, and then in alcohol of 
gradually increasing strength. It is to the use of picric acid as a 
hardening agent that the results obtained are believed to be in large 
measure due. All the more important results have, however, been 
confirmed by specimens hardened in chromic acid in the usual manner, 
though such specimens have almost invariably proved inferior in dis- 
