322 RANDOLPH. [Vol. VII. 



cases of normal division I think the separation is probably 

 brought about by violent muscular contraction. Such division 

 sometimes takes place while the worm is executing its charac- 

 teristic snake-like swimming movements, and under these cir- 

 cumstances it appears to suddenly fall apart. 



The contraction is most marked in the longitudinal muscles, 

 and the effect is to draw over at their free ends the other layers 

 of the body-wall and of the wall of the alimentary canal to which 

 they are attached. The outer wall is curved inward, and the 

 wall of the intestine outward, so as to almost or quite shut in 

 the coelomic cavity of the end somite. The flow of blood from 

 the broken ends of the vessels is very quickly checked, a result 

 possibly of the great contraction which may be imagined to 

 extend also to the walls of the blood vessels. 



The processes of growth begin very soon, and in many cases 

 the new tissue is sufBciently developed to be seen after the 

 lapse of a few hours. 



III. The Formation of Embryonic Tissue. 



By the contraction of the muscles, the posterior edges of the 

 walls of the coelomic cavity of the last somite are brought close 

 together, the ectoderm approaching the entoderm. 



I. Ectoderm and Entoderm. 



The next step in development seems to be simple prolifera- 

 tion of the ectoderm and entoderm, although in what way the 

 new tissue arises I am unable to state. A union of the new 

 ectoderm and entoderm is very soon established, and at the 

 same time a rapid increase in length takes place.^ 



1 In the regeneration of the tail in Lumbricus, where the process is a very slow 

 one, the union of ectoderm and entoderm takes place only after a considerable 

 growth of each separately. After a period of growth posteriorly, the new ectoderm 

 begins to invaginate to form the proctodeum. It grows in from the side, as is the 

 case also in Lumbriculus, and for a considerable time ends bhndly, sometimes 

 dividing into two or more branches, which extend obliquely in different directions. 

 It seems as if union -were established between the new entoderm and that branch of 

 the proctodeum with which it first comes into contact. In consequence of this mode 

 of growth, transverse sections of early stages sometimes show the proctodeum and 

 the new entodermic tube cut at a different level, the two having passed each other 

 without coming together. 



