STRUCTURE OF DOUBLE MONSTERS IN THE EARTHWORM. 347 



unlike all other cases, there are three distinct commissural strands 

 joining it to the nerve cords, one extending over the top of the 

 pharynx, the other two below the point of union of the two 

 digestive tracts forming a triangle with each other. Fig. 19, 

 Plate III., is a cross section of these last mentioned strands and 

 their connection to the two cords. As in No. 70 there is in 

 this monster also a limited connection between the two cords. 



5. Cases in which the two individuals are extremely unequal 

 in size, e. g., bud formation. Thus far there have been found 

 two such cases, a section of one being shown in Fig. 20, Plate III. 



III. DISCUSSION. 



It will be remembered that reference was made to the differ- 

 ence in view concerning the origin of double monsters in an earlier 

 part of this paper. Vedjovsky maintains that the formation 

 of two embryos from one egg is abnormal, whereas Kleinenberg 

 maintains the opposite. A difference of this kind may be ex- 

 plained on the basis of the difference in the forms worked on by 

 these two investigators. This paper is a study of a form very 

 similar, in its mode of development, to that described by Kleinen- 

 berg. It has been found that it is quite common, not only for 

 monsters to appear in this group, but also for two individuals to 

 emerge from one capsule. Out of the 184 cocoons opened 57 

 contained only one individual, 101 two, and 25 eggs in various 

 cleavage stages and i four embryos. Thirty-five of the 101 

 cases were in the form of monsters. Six cases have been noted 

 in which two or more eggs were present but in four of these only 

 one was undergoing development while the others were in various 

 stages of degeneration. One egg was found which had just 

 reached a stage in development sufficiently far advanced to 

 show the division into two hemispheres and the connecting 

 band of larger cells very similar to Kleinenberg's Fig. 6. Three 

 others were found which had begun to elongate in a manner very 

 similar to the above. 



It would certainly be 'safe to conclude that it is a common 

 occurrence for one egg of this species to give rise to two indi- 

 viduals. If this were not the case it would seem rather unusual 

 to find so large a number of cases in which two and only two 



