304 DR. p. MANSON ON THE DETELOPMENT 



On the Development of Mlaria sanguinis Jiominis, and on the 

 Mosquito considered as a Nurse*". By Pateick Maistson, 

 M.D. (Communicated by Dr. Cobbold, F.E.S., T.L.S.) 



[Eead March 7, 1878.] 



Development cannot progress far in the Host containing tTie Parent 

 Worm. — Fortunately it is an almost universal lavy, in the history 

 of the more dangerous binds of Entozoa, that the egg or embryo 

 must escape from the host inhabited by the parent worm before 

 much progress can be made in development. "Were it possible 

 for animals so prolific as Filaria immitis of the dog, or Filaria 

 sanguinis of man, to be born and matured and to reproduce their 

 kind again in an individual host, the latter would certainly be 

 overwhelmed by the first swarm of embryos escaping into the 

 blood, as soon as they had made any progress in growth. If, for 

 example, the brood of embryo Filarice, at any one time free in the 

 blood of a dog moderately well charged with them, were to begin 

 growing before they had each attained a hundredth part of the 

 size of the mature Filaria, their aggregate volume would occupy a 

 bulk many times greater than the dog itself. I have calculated 

 that in the blood of certain dogs and men there exists at any 

 given moment more than two millions of embryos. Now the 

 individuals of such a swarm could never attain any thing ap- 

 proaching the size of the mature worm without certainly involving 

 the death of the host. The death of the host would imply the 

 death of the parasite before a second generation of Filarics could 

 be born, and this, of course, entails the extermination of the 

 species ; for in such an arrangement reproduction would be 

 equivalent to the death of both parent and offspring, an anomaly 

 impossible in nature. 



The Embryo must escape from the original Host. — It follows 

 therefore that the embryo, in order to continue its development 

 and keep its species from extermination, must escape from the first 

 host in some way. After accomplishing this it either lives an 

 independent existence for a time, during which it is provided 

 with organs for growth not possessed by it hitherto ; or it is 

 swallowed by another animal which treats it as a nursling for 

 such time as is necessary to fit it with an alimentary system. 

 The former arrangement obtains in the Filarice inhabiting the 



* [Throughout this memoh- Dr. Manson employs the term "nurse" in the 

 same sense as that in which helminthologists use the term " intermediate host." 

 — T. S. Cobbold.] 



