40 
seen in the clearest manner to terminate as 
they pass upwards towards the eye in well- 
defined cut ends, and I think no one who 
studies these preparations can doubt that in 
them the asters are true fibrillar structures. 
We may now inquire in what manner the 
rays arise and grow, and what is the origin 
of their substance. In the growing aster 
Jen. 
Fia. 3.—(a) Protoplasm and yolk-spheres from the 
ege of Thalassema in section. The upper part of the 
section shows the result of prolonged extraction of 
the dye (iron-hematoxylin) ; the lower half represents 
varying degrees of extraction (1,200 diameters); (0) 
ege of Nereis in section showing yolk-spheres and the 
first polar amphiaster above (600 diameters). 
the rays progressively extend themselves 
from the center outwards, gradually losing 
themselves in the general meshwork. It 
has been maintained by some writers that 
the rays grow outwards from their bases 
SCIENCE. 
[N. S. Vou. X. No. 237. 
like the roots of a plant, and in a certain 
sense this is undoubtedly true. But it is 
difficult to believe that all of the material 
of the rays comes from the base, i. ¢., from 
the nucleus or the centrosome, for they 
often extend themselves throughout the en- 
tire cytoplasm, even in cases where, as in 
the sperm-aster of echinoderms, the center 
of the aster remains very small, and the 
nucleus still consists of a compact mass of 
chromatin (Fig. 4). It is more probable 
that they grow at the tip, continually ex- 
tending themselves at the cost of the ma- 
terial lying in the meshwork. When the 
rays are followed out peripherally they may 
often be seen to run out into rows of gran- 
ules like beads on a string. Van Beneden, 
who has been followed by many later 
writers, was inclined to regard the rays as 
essentially rows of microsomes strung to- 
gether by a homogeneous, clear substance, 
i. e., by the continuous substance, and I 
was led to the same conclusion in the case 
of sea-urchin eggs. A study of the asters 
in Ophiura throws doubt upon this conclu- 
sion, for it is here certain that the larger 
and deeply staining microsomes do not 
build up the ray, but are quite irregularly 
scattered along its course. The rays here 
mainly arise, I believe, in, and at the ex- 
pense of, the continuous substance, and the 
linear arrangement of the microsomes is 
incidental to the differentiation of this sub- 
stance along a definite tract which more 
or less involves the microsomes as it pro- 
gresses. This conclusion probably also ap- 
plies to other forms. The material active in 
the ray-formation appears to be the contin- 
uous substance, and, while the microsomes 
may, and probably in many cases do, con- 
tribute to the ray, they probably play the 
part of reserve material rather than of ac- 
tive elements.* 
* As already pointed out, we cannot assume that 
the ray is merely an accumulation of the continuous 
substance on account of its different staining capacity. 
