536 



NATURE OF REPRODUCTION. 



Fig. 171. 



sexes manifests itself. This is, indeed, more or less the case with 

 all animals and with all vegetables. The blossom, which is the 

 sexual apparatus of the plant, does not appear, as a general rule, 

 until the growth of the vegetable has continued for a certain time, 

 and it has acquired a certain age and strength. Even in the human 

 subject the sexual organs, though present at birth, are still very 

 imperfectly developed as to size, and altogether inactive in func- 

 tion. It is only later that these organs acquire their full growth, 

 and the sexual characters become complete. In very many of the 

 lower animals the sexual organs are entirely 

 absent at birth, and appear only at a later 

 period of development. 



Now the encysted or sexless entozoa are 

 simply the undeveloped young of other para- 

 sites which propagate by sexual generation; 

 the membrane in which they are inclosed 

 being either an embryonic envelope, or else 

 an adventitious cyst formed round the para- 

 sitic embryo. These embryos have come, in 

 the natural course of their migrations, into 

 a situation which is not suitable for their com- 

 plete development. Their development is 

 accordingly arrested before it arrives at matu- 

 rity ; and the parasite never reaches the adult 

 condition, until removed from the situation in 

 which it has been placed, and transported to a 

 more favorable locality. 



The above explanation has been demon- 

 strated to be the true one, more particularly 

 with regard to the Tsenia, or tapeworm, and 

 several varieties of Cysticercus. The Tsenia 

 (Fig. 171) is a parasite of which different species 

 are found in the intestine of the human subject, 

 the dog, cat, fox, and other of the lower animals. 

 Its upper extremity, termed the " head," con- 

 sists of a nearly globular mass, presenting upon 

 its lateral surfaces a set of four muscular disks, or " suckers," and 

 terminating anteriorly in a conical projection which is provided 

 with a crown of curved processes or hooks, by which the parasite 

 attaches itself to the intestinal mucous membrane. To this " head" 

 succeeds a slender ribbon-shaped neck, which is at first smooth, but 



