THE ORIGIN OF NEW CELLS AND TISSUES 195 



58, A). The new cells appear to be lymph cells. Although this 

 mass of cells may be quite large, the cells do not seem to form 

 later any of the organs in the new head. The presence of these cells 

 makes it very difficult to work out the origin of the other cells that 

 appear later. Owing to the absence of this lymph plug in lum- 

 briculus and nais it is easier to follow in them the regenera- 

 tive processes. In the midst of these lymph cells spindle-liker cells 

 soon appear whose origin is obscure, but Hescheler thinks it im- 

 probable that they are transformed lymph cells, although they are 

 completely intermixed with the latter. The spindle-cells arrange 

 themselves later in regular bands, that appear to be extensions of the 

 longitudinal muscles. A few days after the operation, the lymph 

 plug is covered over, beginning at the edge, by the ectoderm. The 

 new ectodermal cells arise from the old ectoderm, and seem to extend 

 over the lymph plug by a sort of migration process. Division of the 

 cells does not occur at this time. These covering cells are at first all 

 alike, the characteristic gland cells of the ectoderm being absent. 

 The digestive tract withdraws somewhat from the outer cut-surface, 

 and its end closes. The closed end abuts against the inner surface 

 of the lymph plug. The next changes are initiated by the appear- 

 ance of karyokinetic divisions in all the tissues of the new part, which 

 lead to a rapid growth and elongation. Dividing cells are found in 

 the new, as well as at the border of the old, ectoderm, where the 

 new and the old parts are continuous. At this stage there appears 

 in the lymph plug another kind of cell, that seems to arise, in part 

 at least, from the ectoderm by an in-wandering of new cells. Other 

 new cells may come from the edge of the old muscles, but it is 

 not clear whether they come from a transformation of muscle cells, 

 or from undifferentiated cells lying in the old muscles. In addition to 

 these sources of new cells, it appears not improbable that cells may 

 separate from the end of the digestive tract. 



Nerve fibres push out from the end of the ventral nerve-cord into 

 the new part, and groups of cells, often in process of division, appear 

 in the old ganglia, even in those that lie a long distance from the anterior 

 end. It is not improbable, Hescheler thinks, that new cells, as well 

 as fibres, grow forward from the most anterior end of the nerve-cord 

 into the new part. A mass of nerve cells and fibres appears in front 

 of the old nerve-cord, and extends upwards and around the digestive 

 tract, to meet over the anterior end of the latter in another mass of 

 cells that have arisen from an early in-wandering of ectodermal cells. 

 It is not improbable that the masses around the digestive tract (the 

 commissures) and also the new ventral cord may also include cells 

 that have had the same origin. 



A tubular invagination of ectoderm is formed at this time at the 



