CESTOID WORMS. 113 

defined, become more distinct, and as they recede from the head 
first, the reproductive organs gradually develop, then the process of 
egg formation is arrived at, the segment still enlarging, the uterus 
enlarging also, until the branched form of this organ disappears in 
the cramming it has undergone; finally the segment appears as a 
‘sac completely filled with eggs. It is now mature; has become 
the largest and the terminal one of the colony, and finally it drops 
off, passes along the bowel of the host, and leaves the same along 
with the foeces; and those small flat worms, often seen on the 
excreta of the dog, are these detached members of the tape-worm 
community. The eggs they contain are either expressed at the 
genital pore during the contortions caused by exposure to a lower 
temperature, or they are liberated ex masse at the decomposition 
of the segment. 
To follow on the cycle it is necessary that the cntermediary 
bearer shall devour these eggs,—should an unsuitable animal do 
so they would be inert ; but, arrived in the stomach of a proper 
bearer, the egg coverings are quickly dissolved by the digestive 
juices, and the embryo, a minute bladder-like creature with six 
hooks, is set free ; at once it attacks the small blood vessels, pierces 
them and enters the circulation. In experiments which have been 
made it has been found that a drop of blood taken from an animal 
twenty-four hours after eating the eggs, that it contained many 
embryos ; carried thus by the circulation of the blood, the six- 
hooked embryos select a suitable part, reattack the lining of the 
vessel they may then be within, pass through it and into the 
adjacent structures. Their power of selection is great, and very 
constant ; those of one species choosing one, and those of another 
a different habitat, in which to develop up into perfect cysticerci or 
larval teenize,—thus it is that the Cysticercus cwnurus is found in 
the brain; the Cysticercus pisiformis generally beneath the perito- 
neum; the Cysticercus cellulose and Cysticercus bovis being 
respectively the pork and beef “ measle” in the voluntary muscles 
of the bearer. Here the process is—first, the hooklets drop off ; 
the cyst enlarges, then at one point it becomes invaginated, much 
after the manner of inverting the finger of a glove. From the 
bottom of the pit thus formed a head is gradually developed, and 
In a varying period becomes complete ; a single head resulting 
from one embryo. This is the usual condition; but in at least 
two instances a much more complex arrangement obtains: First, 
in the larval Tenia cenurus, the egg results in a single embryo. 
This usually locates itself in the brain substance, and a cyst is 
formed; but instead of a single invagination of its wall a large 
number are made, and from the base of each a head is developed, 
and as many as from two to three hundred heads have been 
counted. The second exception occurs in the larval Z: Echino- 
