534 NATURE OF REPRODUCTION. 



is simply this, that if the animal or vegetable germ be deposited in 

 a locality which affords the requisite conditions for its development, 

 it becomes developed ; otherwise not. Each female Ascaris pro- 

 duces, as we have stated above, many thousands of ova. Now, 

 though the chances are very great against any particular one of 

 these ova being accidentally transported into the intestinal canal of 

 another individual, it is easy to see that there are many causes in 

 operation by which some of them might be so transported. By far 

 the greater number undoubtedly perish, from not meeting with the 

 conditions necessary for their development. One in a thousand, or 

 perhaps one in a million, is accidentally introduced into the body 

 of another individual, and consequently becomes developed there 

 into a perfect Ascaris. 



The circumstance, therefore, that particular parasites are confined 

 to particular localities, presents no greater difficulty as to their 

 mode of reproduction, than the same fact regarding other animal 

 and vegetable organisms. 



Neither is there any difficulty in accounting for the introduction 

 of parasitic germs into the interior of the body. The air and the 

 food offer a ready means of entrance into the respiratory and 

 digestive passages ; and, a parasite once introduced into the intes- 

 tine, there is no difficulty in accounting for its presence in any of 

 the ducts leading from or opening into the alimentary canal. Some 

 parasites are known to insinuate themselves directly underneath 

 the surface of the skin ; as the Pulex penetrans or " chiggo" of 

 South America, and the Ixodes Americanus or " tick." Others, 

 like the (Estrus bovis, penetrate the integument for the purpose of 

 depositing their eggs in the subcutaneous areolar tissue. Some 

 may even gain an entrance into the bloodvessels, and circulate in 

 this way all over the body. Thus the Filaria rubella is found alive 

 in the bloodvessels of the frog, the Distoma hsematobium in those 

 of the human subject, and a species of Spiroptera in those of the 

 dog. It is easy to see, therefore, how, by such means, parasitic 

 germs may be conveyed to any part of the body ; and may even be 

 deposited, by accidental arrest of the circulation, in the substance 

 of the solid organs. 



The most serious difficulty, however, in the way of accounting 

 for the production of parasitic organisms, was that presented by the 

 existence of a class known as the encysted or sexless entozoa. These 

 parasites for the most part occupy the interior of the solid organs 

 and tissues, into which they could not have gained access by the 



