CmUij Mdrine Lahorafort/, S(. An<Irpir<i. 77 



the mesoderm forming the outer wall of the rhvnehoeoclom 

 should hifurcate. 



"As to the cause of such an abnormality, several alternatives 

 liavc been sugfjested by Williamson* in discussing a similar 

 ease which occurred in a specimen of Lumbricus. The 

 alternative suggestions arc : 



" (1) Embryonic formation. 



(2) Median division. 



(3) A lateral wound. 



"Except the doubtful case of a Poh/ffordiusAarva. cited by 

 Kitter, there is little evidence to support the first view, 

 though the experimental jjroduction of spina bifida in frog- 

 embryos may be mentioned here (Morgan). 



"The second view may be rejected, owing to the extreme 

 improbability of au injury ever passing exactly through the 

 median line. 



" Williamson himself leans to the third view, which supposes 

 that the gut is damaged, forming a new and shorter passage 

 for faeal matter. Irritation is set up at the injury, resultin" 

 in cell-proliferation and a downward growth of the various 

 organs into tlie bud. Such a view would explain the arrange- 

 ment of the nerve-cords and neurocliords ; but it is dithcult 

 to understand why a medially-situated organ, such as the 

 rhynchocijeloni or dorsal vessel^ should grow down. More- 

 over, such an explanation will not account for many eases in 

 other groups, such as a bifid tail in Lacerta, where the gut 

 ends before the point of injury. 



" The supposition that it is due to an ordinary transverse 

 ])reakage, under certain unknown conditions, seems as feasible 

 as any other explanation. It must be assumed that this 

 occuned behind the proboscis, Init in front of the posterior 

 end of tlie rhynchocaidom, in order to account for the bifid 

 condition of the latter j. 



" Such a view receives a certain amount of support from 

 some recent experiments of T. 11. ^lorgan, who investigated 

 the regeneration of the earthworm after transverse section. 

 In three eases the worm produced a bifid tail (with two 

 vents) on regeneration ; but nothing is said about the 

 arrangement of the internal organs. 



♦ Ann. k Mag. Nut. Ili-t. ser. 6, vol. xiii. p. 217. 



t " Tliid view, of course, leads oii<; to regard the iuner nerve-cord of 

 each of the two ' tails ' us a .structure which lia.s arisen de novo, and not in 

 continuity with the original nerve-cords. The ab.sonce of neurochords 

 in tlieni is not oppo.-'ed tocuch a view, tliough perliaj)-! the * e.xcrescence ' 

 idea is more ia keeping with the little thai is actually known." — li. C. P. 



