REGENERATION 



433 



with stored nutrient material. In their final stages these full-grown 

 neoblasts migrate ventrally down the septa and then posteriorly along 

 the nerve cord through the ventral openings in the septa to the cut end 



dbv 



chl 



....T^-*"*: 



.«««»' 



:*r«"' 





cgv....-' 





si 



I 





•feWfegs-- 





"Vwiis^^ 



X 



Cot) 



epi 



vbv 



-«•■>«« 



nc 



ph 



gnl 



dbv 





ojb 



V 





X 



(c) 



nc 





in+ 





Fig. 13. — Effect of X-rays upon anterior regeneration of Tubifex tubifex. Top — sagitta 

 section of anterior end of control 8 days after removal of three to four anterior segments. 

 Middle — same of a control 18 days after such a removal. Bottom — sagittal section healed 

 anterior end of an X-rayed specimen 30 days after such a removal, eg, cerebral ganglion; 

 chl, chloragogue layer; dbv, dorsal blood vessel; epi, epidermis; gnl, granular leucocytes; 

 int, intestine; nc, ventral nerve cord; ph, pharynx; pro, prostomium; st, stomadaeum; vbv, 

 ventral blood vessel; xx, plane of the cut. {From Stone, 59.) 



of the specimen which has in the meantime become healed by rearrange- 

 ment of the old tissues in advance of any cell proliferation (Fig. 11). 

 Only when a neoblast has reached this posterior region does it divide. 

 From the neoblasts thus accumulating posteriorly there arises by mitosis 

 a mass of smaller cells which differentiate into the new mesodermal 



