W. K. Brooks—Rhythmical Character, etc., of Segmentation. 293 
No. I1.—The Rhythmical Character of the Process of Segmenta- 
tion; by W. K. Brooks. 
A number of observers have called attention to the fact that 
in certain animals the segmenting eggs pass through alternating 
stages in which the segmentation products are first conspicuous 
and well defined, and then flattened and fused together. 
n a paper on the development of the fresh water pulmo- 
nates I have attempted to show that the alternation is due to 
the fact that periods of segmenting activity alternate with 
periods of rest, and that the tendency which the elasticity of 
the egg exerts to render its form spherical when no other force 
is acting upon it causes the partial obliteration of the outlines 
of the spherules during each resting stage. 
The essential factor is therefore the alternation of rest with 
activity, and’ the change of shape during the resting periods is 
a secondary phenomenon, brought about incidentally by the 
physical properties of the yolk. 
In most eggs the yolk is not sufficiently elastic to allow any 
great change of form, but careful time-records show that 
; 
periods of active change alternate with longer periods during 
which there is no external change. 
During the past year various members of the Biological De- 
partment of the University have observed this alternation in 
various Vertebrate and Invertebrate eggs. Dr. Clarke has 
noticed it in an amphibian, Amblystoma, where the segmenta- 
tion is total. I have observed it in the egg of an unknown fish, 
where segmentation is restricted to a blastoderm. Mr. Wilson has 
observed it in three annelides, where segmentation is total and 
irregular; Arenicola, Clymenella and Lwmbricus. It is very 
well marked in an arthropod, Leucifer, whose eggs undergo 
total regular segmentation. 
Its occurrence in so many widely separated groups, with 
such different methods of segmentation renders it probable that 
it will be found in nearly all eggs upon sufficiently careful 
examination. 
