ZOOLOGY: H. E. JORDAN 
149 
both satellites at the times of Jupiter's opposition in 1914 and 1915 and 
of the Ninth SatelHte in 1916. The corresponding directions of the earth 
are shown by the arrows, and the relative positions of the earth and the 
two satellites explain how in 1914 the new satellite was found on the 
same plate as the Eighth. The orbits of the Sixth and Seventh Satel- 
lites, which are also shown in the figure, are drawn to scale but have 
not been projected into the plane of the figure. 
The perturbations produced by the sun are very large for both satel- 
lites on account of their great distance from the primary. The orbits, 
therefore, are not even approximate ellipses; their points of greatest 
and least distance from Jupiter are indicated. It is of course impos- 
sible from the data now available to say much about the mean ele- 
ments of the Ninth Satellite, but the mean period is likely not far from 
745 days. The eccentricity appears to be a little less than that of the 
Eighth SateUite. 
The magnitude of the Ninth Satellite was found by Mr. Shapley 
to be 18.3 on October 18 and 19, 1916,^ which corresponds to 18.6 at 
mean opposition. With reasonable assumptions for the albedo and 
color index, this would indicate a diameter of about 15 miles. As seen 
from Jupiter the satellite at full phase would be between the 11th and 
12th magnitudes, depending on its distance from the planet. 
^Nicholson, S. B. Berkley, Lick Ohs. Univ. Cat. Bull., No. 272, 1915. 
^Crommelin, A. C. D., London, Mon. Not. R. Astr. Soc, 71, 1910, (50-62). 
3 Nicholson, S. B., and Shapley, H., Pub. Astr. Soc. Pac, San Francisco, 28, 1916, 
(281-282). 
AORTIC CELL CLUSTERS IN VERTEBRATE EMBRYOS 
By H. E. Jordan 
DEPARTMENT OF ANATOMY. UNIVERSITY OF VIRGINIA 
Communicated by A. G. Mayer, December 1, 1916 
Aortic cell clusters in mammals were first described by Maximow^ 
(p. 517) in rabbit embryos. Minot^ (p. 523) subsequently described 
similar structures in human embryos of from 8 to 10 mm. length and 
in rabbit embryos. EmmeP reported aortic cell clusters in rat em- 
bryos, rabbit embryos, and in pig embryos of from 6 to 15 mm. 
Jordan^ discovered these clusters in pig embryos (10 to 12 mm.) at 
about this same time, and reported their presence also in mongoose 
and turtle embryos. EmmeP later published a detailed description of 
the aortic clusters of the pig embryo. Meanwhile I had observed 
them also in chick embryos of 3 to 4 days' incubation. Dantschakoff^ 
