442 F. H. Kretkcr, 



growth and where tlie tongue of new tissue joined tlie older portion 

 of the body the fibers converged and individual fibers became deta- 

 ched to inigrate along the wall into the new portion. Some also wan- 

 dered freely in the coelom. In Figure 24 a transverse Strand of such 

 fibers together with some connective tissue extend across the base 

 of the tongue. Towards the anterior end of the body the layer of 

 fibers along the wall becomes thinner and at the tip only a few are 

 present. 



The growth in length of the body of Tuhifex proceeds very slowly 

 and consequently the formation of the new musculature is not rapid. 

 The rate of growth varies, of course, with the level at wliich the worm 

 has been cut. About five days after the Operation when the regene- 

 rating portion of the nervous system has assumed definite shape and 

 the body wall has begun to elongate the new musculature begins to 

 form. Beginning with the portion nearest to the old tissue some of 

 the spindle shaped cells become arranged lengthwise, parallel with 

 the longitudinal axis of the body, along the inner surface of the new 

 ectoderm; at first they are arranged rather loosely, later more com- 

 pactly (Fig. 25 and 26). Upon the border of the cells thus arranged 

 the contractile substance again begins to appear and all the cells are 

 finally knit together in a compact mass of muscle (Fig. 27). As de- 

 velopment proceeds other cells are added to those originally present 

 until the normal thickness of the muscular coat is attained. The mus- 

 cles lining the prostomimn are the last to form. In specimens killed 

 three weeks after the Operation the musculature of this region was 

 still in a very undeveloped state. 



What has thus far been said apjDlies only to the formation of the 

 longitudinal muscles. As at the posterior end, the circular muscles 

 are formed from the basal ends of the ectoderm cells, which remain 

 in situ, by a redifferentiation of the cytoplasm into contractile sub- 

 stance. This change occurs after the longitudinal muscles have begun 

 to form and proceeds from the old portion toward the anterior end 

 of the body (Fig. 26 and 27). A Splitting off of special cells from 

 the ectoderm, as described in the formation of the circular muscle at 

 the posterior end, was not observed at the anterior end. Hepke and 

 V. Wagner also found the circular muscles to be formed from the 

 ectoderm but they maintained that the ectoderm cells first enter the 

 coelom and tlien become arranged about the sides of the coelom before 

 forming the muscle. As stated before a migration of the ectoderm 

 into the coelom was jiot found by the wi'iter. Even if we assume that 



