THE SARCODINA 223 



animal reproduces itself by fission. The sexual process begins by two such 

 amoebae coming together and surrounding themselves with a cyst in common. 

 Within the cyst their nuclei first give off vegetative chromidia, which are 

 absorbed, after which the two nuclei in each separate amoeba fuse together 

 to form a single nucleus, a synkaryon. The protoplasmic bodies of the two 

 amoebae now fuse completely into one, after which each synkaryon goes 

 through two reducing divisions, producing each two reduction-nuclei, of 

 which the first may divide again, so that there may be in the cyst six reduction- 

 nuclei altogether, which are gradually absorbed. The two persistent synkarya, 

 after undergoing this process of reduction, approach each other, but remain 

 separate, and the amoeba is hatched out of the cyst to begin its vegetative 

 life with two nuclei representing gamete -nuclei that have undergone reduction 

 that is to say, pronuclei which remain separate and multiply by fission 

 throughout the vegetative life, and do not undergo syngamic fusion until the 

 end of it. 



In Amoeba binudeata, described by Schaudinn, the vegetative phase also 

 contains two similar nuclei which multiply simultaneously by division each 

 time the animal divides ; but in this case the complete life-cycle is not known. 



Owing to the practical importance of the entozoic amoebae, and 

 the attention that has been directed to them in consequence, their 

 life-cycles have been more studied and are better known than those 

 of the free-living species. According to Mercier, Entamosba blattce 

 multiplies by binary fission in the gut of its host, and later becomes 

 encysted, passing out of the body of its host in this condition. 

 Within the protective cyst it breaks up by multiple fission, follow- 

 ing repeated division of the nucleus, into a number of amcebulae, 

 which are set free from the cyst when it is devoured accidentally 

 by a new host. The amcebulae are gametes which copulate after 

 being set free, and the zygote grows into the ordinary vegetative 

 form of the amoeba. E. blattce thus furnishes a very characteristic 

 and primitive type of the life-cycle of an entozoic amoeba, and one 

 which differs only in points of specific difference from that of 

 Amoeba minuta, described above. 



The question of the human entozoic amoebae is at present in a 

 somewhat confused state. The occurrence of amoebae in the hinder 

 region of the human digestive tract, especially the colon, has long 

 been known, and the name Amoeba coli was given by Losch to such 

 organisms (synonym, Entamceba hominis, Casagrandi and Barba- 

 gallo). It is, however, certain that more than one species of amoeba 

 occurs in the human bowel, and Losch's name must therefore be 

 restricted to one of these. 



An epoch in the study of human entozoic amoebae was marked by the 

 researches of Schaudinn (131), who distinguished two species. The first, to 

 which he restricted the name Entamoeba coli, occurs commonly in Europe 

 and elsewhere as a harmless inhabitant of the intestine that is to say, like 

 E. blattce and many others, it is not, under normal circumstances at least, a 

 parasite in any sense of the word, but a simple scavenger, feeding on bacterial 

 and other organisms, detritus, etc., in the colon and rectum. The second 

 species, to which Schaudinn gave the name E. Mstolytica, * is, on the contrary, 



* Liihe has proposed to place E. histolytica in a separate genus, Poneramceba 

 n. g. (Schr. Physik. Ges. Konigsberg, vol. xlix., p. 421). 



