32 ON PARASITES. 



It, as well as the placenta, is an apparatus developed to meet a cer- 

 tain purpose, and that purpose accomplished it atrophies and is 

 removed. 



If the caudal vesicle were a monstrosity it could not be so uniformly 

 produced as it is. Monstrosities are rare exceptions and not a rule. 



But the human organism affords further analogies. At one time 

 the foetal liver is the largest organ in the body, — later it assumes a 

 subordinate position. During foetal life the thymus gland is quite large 

 and highly vascular. After birth it atrophies and often disappears. 



At one time the function of kidneys is performed by the Woolfian 

 bodies, by and by these atrophy and disappear, whilst the true kidneys 

 simultaneously develop and take their place. Hence it is shewn that 

 the requirements of the human organism at any time are met by the 

 development of organs to meet those requirements, and when they 

 have discharged their function and are no longer required they assume 

 a subordinate position or disappear, whilst new ones take their place. 

 The same principle holds good among the Entozoa. The embryo of 

 the frog at one time is as simple in structure as any entozoon. After 

 a time it becomes a tadpole, breathes by gills like a fish, and with its 

 enormous tail sculls through the water. Later, its caudal appendage 

 and gills atrophy, lungs take the place of the latter, feet and legs that 

 of the former, and the fish has become a reptile. Yet it is the same 

 offspring of the same egg. 



If the batrachian ovule were deposited on the land it would never 

 develope a tadpole ; cast into the outer world the human ovule soon 

 perishes. 



The preceding considerations demonstrate the existence of a typical 

 force from within, that in necessary connection with external circum- 

 stances projects into existence whatever organ is required for the 

 accomplishment of a function. 



Turning now to the Entozoa we are prepared to find them obeying 

 the same laws that regulate the development of the higher animals. 

 No metamorphoses which they manifest are without an analogy else- 

 where. No greater difference in type exists in the various phases of 

 their development than are seen in the human or batrachian embryo. 



It is in the vicissitudes of their career that they differ chiefly 

 from the higher animals. They have to make migrations both active 

 and passive to find the suitable conditions for development. Hence 

 arises the enormous difficulty of following them continuously shewing 



