ASTRONOMY: H. SHAPLEY 
267 
7. The temporary atresia of the esophagus in the Caretta embryo 
would appear to be a device for the protection of the lung during its 
development against yolk material from the gut; which material could 
not be digested but would interfere with normal development of the 
lung. 
8. This hypothesis can comprehend and correlate conditions in em- 
bryos of forms with meroblastic, homoblastic telolecithal , and alecithal 
eggs. Where yolk is very abundant as in the meroblastic eggs of fishes, 
reptiles and birds, the atresia is relatively extensive and of longer dura- 
tion; in amphibia the closure is largely of the nature of a stenosis in 
w^hich yolk globules are involved, probably in process of digestion while 
their forward progress is delayed by reason of the constricted lumen. 
In most mammals and in man such mechanism is functionally super- 
fluous, and consequently absent except in slight and variable degree. 
As such it may persist or become accentuated, and produce congenital 
atresia or stenosis of the esophagus. 
A more detailed description will appear in Publication No. 251 of the 
Carnegie Institution of Washington. 
1 Balfour, F. M., A monograph on the development of Elasmobranch Fishes, London, 1878. 
2 Kreuter, E., Solide oesophagus den Selachier, Erlangen, 1903. 
3Kreuter, E., D., Zs. Chir., Leipzig., 79, 1905, (1-89). 
''Dean, B., Q. J. Microoc. Sci., London, 38, 1896. 
^Oppel, A., Vergleichung des Entwicklungs grades der Organe zu verschiedenen Entwick- 
lungszeiten bei Wirbeltieren, Jena, 1891. 
6Meuron, P. de., Paris, C.-R. Acad. Sci., 102, 1886. 
^KoUmann, J,, Lehrbuch der Entwicklungs geschichte des Menschen, Jena, 1898, (1-658). 
8 Lewis, F. T., Keibel, and Mall, Human Embryology, Philadelphia, 2, 1912, (355-368). 
STUDIES OF MAGNITUDES IN STAR CLUSTERS. V. FURTHER 
EVIDENCE OF THE ABSENCE OF SCATTERING 
OF LIGHT IN SPACE 
By Harlow Shapley 
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Communicated by G. E. Hale, February 15, 1917 
The interest and importance attached to an accurate quantitative 
knowledge of the scattering of Kght in space, for all studies of the ex- 
tent and character of the stellar universe, has been commented upon 
by various writers, particularly by Professor Kapteyn. In the first 
communication of this series^ the matter was discussed briefly and 
evidence was presented showing that interstellar space, at least in the 
direction of the Hercules cluster, is free of the kind of light absorp- 
tion that modifies the color of the stars. The generalization of this 
