116 THE AMERICAN MIDLAND NATURALIST. 
FIGURE 5. 
The remarkable spiral nebula, M 51, in the constellation Canum 
Venaticorum, or the Hunting Dogs. This nebula shows with remark- 
able clearness the great central mass from which extend two partly 
coiled arms. In these arms can be seen large knots, which play so 
important a part in the conception of the Planetesimal Hypothesis. 
Photographed at the Yerkes Observatory. From Chamberlin, University 
of Chicago Press. 
