Somc Phoiionion.i of Rct;onora1i(in in Liiniioflrihis aml rolatcd Foriiih;. 44.'> 



tlic waiidering luesodcrm cells froin tiie okl portiuii of the body re- 

 present wliat these authors supposed to bc cells derived from the ecto- 

 derm it does not change matters so far as the forms being considered 

 iti this paper are coiiccriied, since nonc of them were seen being trans- 

 fornied into circnlar nuisck\ Iwanow does not pay particular atten- 

 tion to the circular musclc but the general impression conveyed by 

 his aocount is that he considers all the new mesodermal structures 

 to be fonned from the mesodermal elements already present. In 

 addition to the evidence afforded by his observations Iwanow argues 

 that the formation of the mesoderm at the anterior end from the 

 ectoderm is improbable on embryological grounds since during the 

 development of the oligochaets (especially Lumbricus and Nais) 

 there is no indication of a division of the mesodermal plates into 

 an anterior and a posterior primordium, the former to give rise to a 

 certain immber of somites and the latter to form all the rcniainiiig 

 somites. 



When (nie conipares the origin of the urgaiis at the anterior end 

 witli that of those at the ])osterior end it is found that practically the 

 only point of difference is in the origin of the secondary mesodermal 

 structures and that even this difference is one rather of degree than 

 of kind. The majority of investigators are agreed that at both the 

 anterior and the posterk)r end the nervous system is derived from 

 the ectoderm and that the entodenn is regenerated from the old ento- 

 derm. The origin of the mesoderm is likewise believed by many to 

 be the same at both ends, namely, from the ectoderm. This is partly 

 true in that the origin of the circular muscles is, at both ends, in the 

 ectoderm. With regard to the secondary mesoderm an inrceasing 

 immber of authors, beginning with Randolph, has found that at the 

 posterior end it is derived from the neoblasts, which are reserve meso- 

 dermal cells of embryonic nature. Rievel thought there were indi- 

 cations that the mesoderm at the anterior end is derived from the 

 old mesoderm and the evidence adduced by Iwanow and niyself leads 

 one to the conclusion that the secondary mesoderm at the anterior 

 end of the body is actually derived from the old mesoderm. Thus at 

 the anterior end the new secondary mesoderm by a process of redif- 

 ferentiation is the product of the mesoderm already present wliile 

 at the posterior end it is also a product of the mesoderm already pre- 

 sent not, however, by a process of redifferentiation but by the re- 

 newed activity of embryonic mesoderm, which has been in a state 

 of retarded activity. 



