192 RE GENERA TION 



processes begin, it is probable that the changes are due, in large part, 

 to an out-wandering of ectodermal cells, or, what amounts to the same 

 thing, to the leaving behind of cells as the old ectoderm withdraws from 

 the cut-end. In the new ectoderm over the end, an active process of 

 proliferation takes place (Fig. 57, B), that leads to the production of a 

 large number of cells lying within the new part. The ectoderm has at 

 this time begun to bulge outward, so that the proliferated cells come 

 to lie within the dome-shaped beginning of the new head. There 

 appears to be some difference in the number and in the location of 

 the proliferations in different species. In general, the new cells arise 

 from the ventral and ventro-anterior region of the dome-shaped ecto- 

 dermal covering of the new part. Most of this new material gives 

 rise to the brain, commissures, and ventral nerve-cord (Fig. 57, Q. 

 The cells giving rise to these structures in tubifex come from two ven- 

 tral regions of proliferation that extend along the sides and dorsally to 

 the anterior end in front of the digestive tract. Where the two masses 

 meet above and in front, the brain is formed. 1 The cells that do not 

 take part in the formation of the nervous system give rise to the mus- 

 cles and connective tissue of the new head. These cells lie especially 

 at the outer sides of the proliferated mass. The origin of the new 

 muscles from ectoderm stands in sharp contrast to the current ideas 

 in regard to the origin of new tissues, and yet it is a point on which 

 the more recent investigators are entirely in accord. Michel, Hepke, 

 and von Wagner have arrived at the same conclusion after a careful 

 examination, and there seems to be no reason for refusing to accept 

 their results. The theoretical importance of this discovery will be 

 discussed later. 



Soon after the proliferation from the ectoderm has begun, the 

 blind end of the digestive tract starts to push forward (Fig. 57, Z>). 

 The cells in the most anterior part of its wall begin to divide, and the 

 end grows in an anterior direction as a more or less solid rod. This 

 rod extends, in some species, as far forward as the ectoderm, meeting 

 the latter on the inner side of its antero-ventral surface. At this 

 point an in-turning of ectodermal cells, in the form of a blind pit, 

 develops, and later this pit, deepening to become a tube, forms the 

 mouth cavity. Its inner end is from the beginning in contact with 

 the anterior end of the digestive tract, or else it connects with the 

 latter soon after its formation. The two flatten against each other, 

 the cells draw away in the middle of the region of contact, and the 

 cavity of the new mouth becomes continuous with the cavity of the 

 old digestive tract. The mouth lies at first nearly terminal in posi- 

 tion (Fig. 57, E), but by the forward growth of the body wall over 



1 In some species the two proliferating regions seem to be in contact above from the 

 beginning (Hepke, in Nais~). 



