408 Miscellaneous. 



anal belts of cilia, and a broad ventral band, and becomes a " Telo- 

 trocbous " larva which passes directly into the adult. The setae 

 develop from before backwards ; and those of the dorsal ramus 

 appear before those of the ventral. 



The segmentation is closely similar to that of some Oligochseta 

 (Euaxes, Tubifex), and resembles also that of the leeches. The 

 gastrula stage is not attained by a typical invagination, but by a 

 downgrowth of the ectoderm over the entoderm. — Amer. Journ. 

 Sci. Oct. 1880. 



Beaufort, N. C, July 1880. 



The Rhythmical Character of the Process of Segmentation. 

 By W. H. 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. 



In a paper on the development of the freshwater pulmonates I 

 have attempted to show that the alternation is due to the fact tbat 

 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 the 

 process of segmentation is rhythmical, and that short periods of 

 active change alternate with longer periods during which there is 

 no external change. 



During the past year various members of the Biological Depart- 

 ment of the Johns Hopkins University have observed this alternation 

 in various vertebrate and invertebrate eggs. Dr. Clarke has noticed 

 it in an amphibian, Amblystoma, where the segmentation 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, 

 ClymeneUa, and Lumbricus. 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. — 

 — Amer. Journ. Sci. Oct. 1880. 



