THE ORIGIN OF NEW CELLS AND TISSUES 193 



and in front of the mouth to form the prostomium, the mouth comes 

 later to lie more on the ventral surface. The short tube produced by 

 the in-turned ectoderm forms only a short part of the digestive tract. 

 It leads from the mouth opening to the new pharynx, and forms, 

 therefore, only the buccal cavity. A similar ectodermal tube, the 

 stomodaeum, which develops in the egg-embryo, becomes not only 

 the buccal chamber, but also the lining of the pharynx. The latter 

 is, therefore, considered an ectodermal structure in the embryo. On 

 the other hand, in the regenerated head the lining of the new 

 pharynx arises from the anterior part of the endodermal digestive 

 tract. We find, therefore, that the same organ, the pharynx, may 

 arise in the same animal from distinct "germ-layers." This result 

 also has an important bearing on our ideas concerning the value and 

 meaning of the so-called "germ-layers," and has helped to bring 

 about a revolution of current opinion as to the importance of these 

 layers. 



The preceding account of the development of the head has shown 

 that while certain of the new organs and layers arise from the same 

 organs of the old part, yet this is not true for all of them. Thus 

 while the ectoderm gives rise to ectoderm, the new muscles do not 

 appear to come from the old ones, or even from other mesodermal 

 tissues, but from the ectoderm. The old digestive tract gives rise to 

 the greater part of the new one, but the new pharynx comes from 

 the old endoderm, and not from the in-turned ectoderm. The nervous 

 system does not arise from the old ventral cord, but from a prolifera- 

 tion of ectoderm. It has, thus, the same origin as the nervous sys- 

 tem of the embryo. The origin of the new blood vessels has not 

 been satisfactorily made out. The seta sacs arise from ectodermal 

 pits as in the embryo. 



In regard to the origin of the new mesoderm, the evidence is still 

 insufficient, I think, to show that cells derived from the old muscles 

 or peritoneum take no part in the formation of the new muscles and 

 peritoneum ; but that the greater part of the new muscles, etc., comes 

 from the proliferated cells can scarcely be doubted. This latter dis- 

 covery loses none of its significance, however, even if it should prove 

 true that the old muscles, etc., contribute something to the new part. 

 It is also not entirely disproven that the ventral nerve-cord does not 

 take a small share in the development of the new cord. 



The regeneration of a new tail-end in these same forms appears 

 to take place in much the same way as the head. The cut-end 

 quickly closes ; later a layer of ectoderm appears over the posterior 

 surface, and the new part bulges out and becomes dome-shaped. 

 A paired, or in some species a single, region of proliferation develops 

 irom the ectoderm, that gives rise to the new ventral nerve-cord. 



