1 1 8 E VOL UTION AND DISEASE. 



carefully studied by Kleinenberg, 1 and he has succeeded 

 in tracing the development of these worms step by step, 

 and shows beyond any doubt that it is a normal condi- 

 tion in L. trapezoides for a single germ to produce two 

 embryos. For a time the embryos are united and turn 

 gently in the albumen without at all impeding each 

 other. The commissure uniting them relaxes gradually, 

 then breaks, and the embryos are freed. There are 

 cases not at all rare in which this singular form of 

 development leads to monstrosity. In fact, among per- 

 fectly developed worms, double monsters occur pre- 

 senting all grades of concrescence, from those firmly 

 united along the whole extent of the body, so that 

 separation is impossible without breaking the embryo 

 into pieces, to others which are hatched coupled together, 

 but only by a thin, frail ligament that the worms easily 

 effect a separation. These junctions are always confined 

 to the epithelial layer of the body wall. 



Kleinenberg states that he has never found one of 

 these eggs giving rise to a single embryo. It is true 

 that a single worm escapes from a capsule, but then 

 nearly always the remains of its companion are found. 

 The accompanying sketches exhibit the extreme forms 

 of these double embryos. In fig. 64 A, they are shown 

 when of equal size ; in B, one of the embryos has 

 undergone suppression. 



We have direct evidence among vertebrata that two 

 embryos may arise from a single ovum. It has been 

 actually witnessed in a batrachian by Clarke. 2 In the 



1 Quart. Journal of Micros. Science, vol. xix. 1879. 



2 "Ann. Mem. Boston Soc. Nat. Hist." 1880. 



