ON PARASITES. 33 



their progress from the egg to the perfect animal. The necessary 

 conditions cannot be inferred, they need to be ascertained by actual 

 observation. All the metamorphic changes which mammalian em- 

 bryos undergo are performed in a single locality, viz., the maternal 

 generative passages. Their identity is always certain. The metamor- 

 phoses of the entozoa occurring in different localities and in widely 

 different organisms, identity is always in doubt. 



Were the case different, — did the mammalian embryo at that early 

 period, when its identity could not be determined from the closest 

 examination, find its nidus for development elsewhere than in the 

 maternal uterus, its embryology would become vastly more difficult. 



Therefore the determination of locality is an important element in 

 the embryology of Entozoa. And often when the right animal has 

 been found for the development of an entozoon a stage, and the crea- 

 ture in the preceding stage has been placed in the desired locality, 

 development does not proceed. Thus Kuchenmeister fed a pig with 

 mature proglottides of the taenia, no cysticerci apppeared in the flesh 

 of the pig. A second pig was fed in a similar manner and still no 

 result followed. Three sucking pigs were fed with proglottides and in 

 due time vast numbers of cysticerci were found in their bodies. 



Hence it is not merely the particular species of animal that must be 

 known to supply the requisite nidus for development, but also one with 

 the proper idiosyncracy, so to speak, or at any rate the proper physi- 

 ological conditions. Of the nature of those precise physiological 

 conditions we as yet are profoundly ignorant. How many men there 

 are who unconsciously swallow live six- hooked embryos and resting 

 scolices, and escape infection we know not. 



Having thus invoked the aid of comparative physiology to elucidate 

 the subject, I proceed to trace the development of the cestoid entozoa. 



A few notes of additional points in the anatomy of the perfect animal 

 are desirable before proceeding with the subject. 



The Proglottis 



is hermaphrodite. The two sets of generative organs are entirely 

 distinct. The male apparatus consists of a testis, vas deferens, and 

 an intromittent organ, which last, when not in use, is retracted in an 

 inverted manner into a sac. The female apparatus is complicated and 

 peculiar. It consists of one external genital orifice or vagina, — co- 

 pulatory vesicle (receptaculum spermatozoorum), — matrix or uterus, 



VOL. IV. D 



