STRUCTURE OF DOUBLE MONSTERS IN THE EARTHWORM. 34! 



normal. He explains the formation of double embryos as 

 being due to what he terms Doppelfurchung. The egg elongates 

 in the direction of the animal pole and subsequently divides into 

 two blastomeres either of equal or unequal size. They are 

 designated as A and a. From each of these cells there is pro- 

 duced a quartet, one lying upon the other giving the appearance 

 of micro- and macromeres in the cleavage of Crepidula or the 

 eight cell stage of Synapta, depending on the proportionate sizes 

 of A and a. Each of the quartets finally, gives rise to an indi- 

 vidual which remains attached to its companion. If the a cell 

 is smaller than A, there is produced one well developed individual 

 and a bud. 



This author has found examples of monster formation in 

 Lumbricus terrestris, Allolobophora fcetida and in Allolobophora 

 trapezoides. In the first group he found two cases in both of 

 which the two individuals were joined on the dorsal side through 

 one segment. Only one monster was found among several 

 hundred embryos of the second group. In the third group a 

 large number was found which the author describes under three 

 divisions. 



A. Those monsters in which the individuals are united on the 

 ventral side along the entire length of the body (PI. 19, Fig. 14). 



B. The cases in which the members are joined on the dorsal 

 side (Plate 21, Fig. 7). 



C. The double monsters in which the individuals are fused end 

 to end. (a) Those in which both members are of similar length 

 (PI. 21, Fig. 9). (b} Those cases in which one member is rudi- 

 mentary (PI. 19, Fig. 12). 



II. OBSERVATIONS. 



Even though these two workers differ so markedly in their 

 conclusions it will be noted from the following that the observa- 

 tions made by the writer are similar in many respects to those 

 made by both of these men. 



In this study of the monsters of Helodrilus caliginosus trape- 

 zoides it has been found that the greatest variety exists in the 

 structure of the nervous system. There is in every case a 

 complete union of the five layers of the body wall, and the 



