574 REPRODUCTION. 



place in the lungs of the green frog, another in those of the brown 

 frog. Cysticercus cellulosae is found in the connective tissue ; Trichina 

 spiralis in the substance of the muscles. 



With regard to many of these parasites the only difficulty in account- 

 ing for their existence, otherwise than by spontaneous generation, lay 

 in their being confined to such narrow limits. It seemed probable that 

 some local conditions must be requisite for the production of a parasite, 

 which was only to be found in the biliary passages, the kidneys, or the 

 lungs of a living animal. A little consideration, however, makes it 

 evident that these conditions are neither necessary nor sufficient for 

 the production of such a parasite, but only for its development and 

 nutrition. Most internal parasites reproduce their species by genera- 

 tion. They have male and female organs, and produce fertile eggs, 

 often in great abundance. The eggs in a single female Ascaris are to 

 be counted by thousands ; and those in a tapeworm by millions. In 

 order that these eggs may be hatched, and their embryos developed, 

 certain conditions are requisite ; as the seeds of plants need, for their 

 germination and growth, an appropriate soil and a certain degree of 

 warmth and moisture. It is no more remarkable that Oxyuris vermi- 

 cularis should inhabit the rectum, and Ascaris lumbricoides the ileum, 

 than that Lobelia inflata should grow only in dry pastures, and Lobelia 

 cardinalis by the side of running brooks. The lichens flourish on the 

 exposed surfaces of rocks and stone walls ; while the fungi vegetate in 

 darkness and moisture, on the decaying trunks of dead trees. Yet all 

 these plants are reproduced by generation, from germs which require 

 special conditions for their growth and development. If any animal 

 or vegetable germ be deposited in a locality where the requisite condi- 

 tions are present, it is developed and comes to maturity ; otherwise 

 not. Thus the internal parasites, like other living organisms, are con- 

 fined to certain situations by the necessities of their nourishment and 

 growth. 



But in some instances there are two further difficulties. First, the 

 parasites in question do not inhabit the open passages or canals of 

 the body, but lie encysted, in the solid substance of the tissues, with- 

 out visible means of access from the exterior. Secondly, they are sex- 

 less ; and, if they perform no generative function, it does not readily 

 appear how they can themselves have beea derived from parents. The 

 two kinds of entozoa, in which these peculiarities are most strongly 

 marked, and in which they have been must fully explained, are Cysti- 

 cercus cellulose and Trichina spiralis. 



1. Cysticercus cellulosse. Thi.5 is a bladder-shaped parasite of some- 

 what flattened form, about 10 millimetres in diameter, found in the sub- 

 cutaneous and intermuscular connective tissue of the pig, where it 

 appears under the form of whitish specks, giving rise to the appearance 

 known as "measly pork." Each parasite is enveloped in a cyst, but 

 the bladder-like body, when extracted, exhibits at one spot a minute 



