118 EVOLUTION AND DISEASE. 
carefully studied by Kleinenberg,t 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 aseparation. 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 
* Quart. Journal of Micros. Science, vol. xix. 1879. 
* “Ann. Mem. Boston Soc. Nat. Hist.” 1880. 
