122 COMPARATIVE ANATOMY. 



small, fissuration and gemmation occur; but neither are 

 seen in Vertebrata, where the specialization is great. 



The application of these facts to a proper appreciation of 

 the process of repair is extensive. It will be seen that when 

 the reserved force is large that losses sustained by mutilation 

 which ma} 7 be carried to an indefinite extent are at once 

 recovered by the evolution of new individuals from the severed 

 portions; as is seen in Infusoria, Hydra, Actinia, and in a less 

 degree in Lumbricus (earthworm). When the force is smaller, 

 limited portions only of an individual can be restored after 

 such injury. Thus the viscera of Holothuria (sea-cucumber), 

 limbs of Ophiura and Lupa (crab), head of Helix (snail), eye 

 and limb of Proteus (salamander), fin offish and tail of lizard, 

 are the only portions capable of being reproduced; for death 

 may ensue if the injury prove extensive. In Aves and Mam- 

 malia, so perfect is the specialization of function that the 

 power named operates but feebly; instances of its sustained 

 activity are rare.* 



PROTOZOA. E-hizopoda. Generation imperfectly known. 

 Foraminifera increase in size by gemmation. Ova-like 

 bodies have been detected in the same group. (Wallich.) 



Infusoria. Generally propagate by fissuration and gem- 

 mation. Fissuration frequently occurs longitudinally, as in 

 Vorticella, transversely with others, as in Stentor,aud in others 

 again by both methods, as in Paramecium. Gemmation is 

 more rare, as in Vorticella. The 'nucleus* is always present in 

 Infusoria. It may be single, as in Paramecium, or multiple, as 

 in Stylonychia. It is always divided in the act of fissuration. 

 In Vorticelia, in addition to the above methods, encapsulation 

 occurs. The animal about to undergo this process loses its 

 identity by becoming, for the most part, converted into a cap- 

 sule having thickened walls. The nucleus assumes a band-like 

 form, which after a time divides into a number of disciform 

 bodies, giving the capsule a sacculated appearance. Within 



* These truths remind us that it is an error to endeavor to explain too 

 exactly the morbid processes going on in man by what is observed in appa- 

 rently similar conditions among lower animals. 



