^0 ON PARASITES. 



Specialization still going on, a well defined body with organs is 

 evolved, and so through a succession of stages starting from the primal 

 segmentation even man's most complex organism is produced. 



Regarding the influence of surrounding circumstances upon the 

 development of the entozoa, the reflected light from the higher animals 

 must be serviceable. By simply observing the essential difi'erence 

 between the cystic and cestoid worms, many enquirers have felt com- 

 pelled to refer the phenomena to external influence. The similarity 

 of the heads of those two worms clearly indicated that they were of 

 the same animal, and direct experiment proved that a cystic worm 

 when introduced into the alimentary canal of a suitable animal develop- 

 ed into a cestoid, but how to account for the caudal vesicle, present as 

 it was under one set of circumstances, and cast off undei another, 

 was the question. Let us see how far the observed development of 

 some of the higher animals bears upon the question at issue. At first 

 the fecundated ovules of all vertebrata have essentially the same 

 characteristics. Let us follow the history of one in the human female. 

 Having lodged upon the mucous membrane of the uterus that has 

 undergone suitable preparation, pending the arrival of the expected 

 guest, it passes through the stage of segmentation, then successively 

 appear the various organs of the body, and after the suitable pitch of 

 development has been attained, to fit the foetus for another stage of 

 existence, it is extruded from the habitation that it so long had occu- 

 pied into the outer world. Now at one period of its intra-uterine life this 

 foetus was completely destitute of several organs, and of very simple 

 structure generally. It therefore was in the same physiological con- 

 dition as the cercaria, the trichina spiralis, the cystic and other 

 sexless worms, except that the human foetus was securely lodged 

 always in the same locality, in the uterus of the mother, whilst those 

 other animals are in the outer world or in the tissues of animals of 

 different species from themselves. Yet the influence of surrounding 

 circumstances is highly important in developmental history. 



The cercaria which at one time finds its proper element in the water, 

 would perish if transferred too soon into some other medium. The 

 cystic worm if suddenly hurried from its home in the cellular tissues 

 or a serous cavity would also perish. It is needless to say that abor- 

 tion is necessarily followed by the death of the human ovum. 



Again at certain stages of its existence the human embryo has special 

 organs adapted to its requirements at the time. Thus at first it is 



