Some Phenomena of Regeneration in Liinnodrilus and related Fornis. 421 



confused them with the neoblasts. Appearances in this part of the 

 Ixxly are extremely misleading as will be seeii from tlie foUowing de- 

 scription. 



The appareiit migration of the ectoderm into the body cavity in 

 t he septa forming zone is largely due to the plane of section and to the 

 similarity between the ectoderm cells and the neoblasts. The neoblasts 

 and the primordia of the septa are closely applied to the inner surface 

 of the ectoderm and, except whon taken through the median plane, 

 a longisection of a cylindrical object, such as is the body of these worms, 

 will pass through the cylinder in such a way that the neoblasts etc. 

 will be in the central portion of the section, while the ectoderm will 

 appear to be extending from the sides into the coelom and joining the 

 neoblasts (Fig. 13). In reality the ectoderm has merely been cut 

 diagonally. The fact that the ectoderm cells and the neoblasts are 

 almost mirrored images of each other and also the fact that the ecto- 

 derm cells are elongated in the transverse axis of the body together 

 with the important fact that the line of demarcation between the neo- 

 blasts and the ectoderm is frequently very indistinct, all add to the 

 Illusion so that it is often only by the most careful Observation that 

 one can discover the actual relations. 



Except at the point where the ventral nerve cord is regenerated 

 the basement membrane of the ectoderm does not break. It is very 

 thin and frequently the ectoderm cells appear to be entering the coelom 

 bat an examination of such cases with an oil Immersion lens and high 

 power oculars has convinced me that in none of these cases does the 

 ectoderm enter the coelom. Gross sections of the body through the 

 regio n under consideration reveal a bulging out of the ectoderm into 

 the coelom on both sides of the raidventral line at which point neoblast- 

 like ectoderm cells seem to be entering the coelom (Fig. 15). These 

 projections are the primordia of the ventral nerve cord and the cells 

 composing them enter the coelom to form the nerve only and do not 

 form mesoderm. 



The manner in which the neoblasts give rise to the mesodermal 

 structures has been so well described by others that only a brief sketch 

 of the process need be given here. In the normal individual neoblasts 

 of what may be termed adult size or nearly so are attached to the 

 septa at various points along the ventral side of the body and shortly 

 after an individual's body has been severed the larger neoblasts detach 

 themselves from the septa and migrate by means of amoeboid move- 

 ments along the nerve cord to the posterior end of the body i. e. to the 



