14 ON PARASITES. 



however powerful can pierce the outer envelope of their digestive 

 passages and permit a view of the digestive process as it actually oc- 

 curs. In impenetrable darkness is that great problem worked out 

 continually, and glimpses of it only, are caught by the experimental- 

 ist, who establishing fistulae at various places in the course of the 

 intestine, withdraws at will the materials in the neighborhood, for 

 purposes of examination. 



The ovaries and testes form ovules and zoosperms, the precious de- 

 positories of the vital principle for perpetuation of species. The 

 highest interest is therefore attached to them. But the action of 

 these organs can only be judged of by their effect, withdrawn from 

 the body from time to time. To trace the successive stages of the 

 formation of ovules and zoosperms in situ, is impossible. 



But in the entozoa, and the simple animals that dwell in the 

 depths of the seas, provision is made for those examinations which 

 in the higher animals are impossible. 



These creatures, passing their lives away from the light, are quite 

 diaphanous. Their simple cellular structure also favors examination. 

 Placed in suitable media, in the living state, all the details of their 

 structure and functions can be examined from the beginning to the 

 close of their existence. They, as it were, invite science to the study 

 of life under sufficiently simple forms for comprehension. Each 

 atom of food may be traced through all the changes that it undergoes. 

 Thus, in the interior of man's own organism, in that very digestive 

 passage whose functions are such a mystery, a structure is formed 

 which will yet serve to explain the very function which produced it. 

 At the culminating point of animal development the simplest living 

 forms appear, and extremes meeting on a common ground, reveal a 

 general law : the identity of digestion throughout the animal scale. 

 Such nematode entozoa as the ascaris mystan (parasite to the cat,) 

 possessing a genital system exactly fitted for the purpose, have served 

 to reveal the entire process of formation in ovules and zoosperms, the 

 impregnation of the former by the latter, and their subsequent history. 

 Placed beneath the microscope they assume a magnitude suitable for 

 examination. It may further be remarked in this place that many 

 of these creatures so slightly disturb the health of the animals they 

 inhabit, and are so constantly present, that the experimentalist soon 

 ceases to regard them as morbid phenomena. 



Are these creatures then unworthy of scientific enquiry ? Let the 



