BIFURCATION IN EMBRYOS OF TUBIFEX. 199 



How universal this high frequency of bifurcation is in the 

 development of Tubifex tubifex in all of the varied conditions 

 under which it lives remains yet to be determined. Furthermore, 

 no statement can be made at this time concerning the relation 

 of bifid embryos to those extremely rare bifid oligochaetes found 

 in nature, although the available evidence leads one to suspect 

 that the former do not survive and that the latter are the result 

 of regeneration following injury. 



MORPHOLOGY. 



Only a preliminary account of the internal structure of these 

 anomalies, based upon serial sections of a few individuals, will 

 be given at this time. All specimens studied thus far show the 

 trunk- portion of each worm to be a double region whose con- 

 struction would be simulated by opposing intimately the cut 

 surfaces of two similar ventral halves of normal embryos of the 

 same degree of development. The branches of the bifurcate 

 portion, however, lack this doubleness, being constructed on 

 the plan of a normal individual. Evidence of the double char- 

 acter of the trunk portion is most readily exhibited in the setae 

 bundles, the nervous system, the nephridia and the principal 

 blood vessels. 



Sections through bifurcate portions of specimens show in each 

 branch four setae bundles arranged in the normal fashion with 

 the component setae so set and curved as to conform with those 

 of normal specimens provided the nerve cord be interpreted as 

 corresponding to the normal ventral one. In the trunk portion 

 of the body eight bundles of setae occur in sets of four, each group 

 being the reverse of the other in arrangement and each being 

 , properly related to one of the two ventral nerve cords. Further- 

 more, the distribution of uncinate and capilliform setae is as this 

 interpretation would require. 



Throughout the trunk portion two nerve cords, 180 degrees 

 apart, extend without change to points where bifurcation appears. 

 If the anterior end be unbranched, a mass of nervous tissue 

 encircles the pharynx; if on the other hand it be branched each 

 branch bears a cerebral mass and esophageal commissures. 



