30 THE PHYSIOLOGY OF TWINNING 



of a degenerate or absorbed twin were usually dis- 

 tinguishable. 



Vejdovsky (1882-92) a few years later made an 

 extensive study of twinning in three other species of 

 earthworms, Lumbricus terrestris, Allolohophora foetida, 

 and Allolohophora trapezoides. Only two twinned speci- 

 mens were found in the first species, two in the second, 

 and large numbers in the third. He found a great 

 variety of conditions which may be summarized as 

 follows: (i) One case in which the components were 

 united on the ventral side along the whole length of the 

 body. (2) Cases in which union was along the dorsal 

 side (frequent). (3) Cases in which union was end to 

 end (rare). In all of these types, except in those where 

 the union was throughout the entire length, there occurred 

 cases of more or less marked size inequality between the 

 two components. Vejdovsky states that completely sep- 

 arated twins occur with extreme rarity in the species 

 studied by him. 



Weber (191 7) made a study of double monsters in 

 the earthworm Helodrilus caliginosus trapezoides. In 

 this species there occur many completely separate twins 

 as well as large numbers of conjoined twins. It is also 

 important to note that in nearly a third of the cases 

 observed an egg gave rise to but a single individual and 

 that a few eggs produced quadruplets. The majority 

 formed double monsters. Since some significance 

 attaches to the manner in which the components of 

 these conjoined twins were united, certain details of 

 Miss Weber's observations must be given. She classifies 

 the double monsters according to their mode of union 

 as follows: (i) Those in which the union is dorsal; 



