B. =Streptococcus Pyogenes and Streptococcus Erysipelatis.=--These two organisms do not differ in morphology nor characteristics, and, while for some time considered as distinct from each other, are now by most observers regarded as identical. The streptococci grow in chains of variable length, and individual cocci vary in size. They grow with and without oxygen, in all media, at ordinary temperatures, do not liquefy gelatin, stain readily, sometimes but not invariably coagulate milk, and vary in longevity. They differ extraordinarily in virulence according to their sources. [Illustration: FIG. 4 Staphylococci in pus. × 1000. (Fränkel and Pfeiffer.)] [Illustration: FIG. 5 Streptococci in pus. × 1000. (Fränkel and Pfeiffer.)] There are many streptococci not included under the above head which are indistinguishable morphologically and in other respects, and yet which are partly or entirely free from pathogenic activity in man. A biological study reveals remarkable and unexplainable transformation between the different members of this species, a part of which may be referable to conditions pertaining to the organisms infected, but part of which apparently pertains to the bacteria. It is held by some that scarlatina is an invasion by certain organisms of this class; this, however, is not yet definitely established. When found in the stools of children with summer diarrheas they are regarded as indicating ulceration of the intestinal mucosa. In contradistinction to the staphylococci, the streptococci manifest a predilection for lymph vessels and lymph spaces, along which they extend with great rapidity. They have less peptonizing power than the staphylococci (except in the absence of oxygen); hence streptococcus infection assumes usually the type of widespread infiltration rather than of circumscribed and distinct edema. One sees remarkable instances of this in cases of phlegmonous erysipelas. It is suggested also that the peculiar manner of growth of the streptococci, in long chains which may coil up and entangle blood corpuscles, has much to do with the formation of fat emboli and with pyemic disturbances. Both these bacterial forms have the power of producing lactic fermentation in milk; and lactic-acid formation sometimes takes place with suppuration in the human tissues, causing acidity of discharge, sour odor, and watery pus. It appears also that these two pyogenic forms have less power of ptomain or toxin formation than many others, and, consequently, that the pyrexia attending suppuration or purulent infiltration is not always to be ascribed to this cause alone, for fever may in some measure be due to tissue metabolism attending their growth, the metabolic products being pyretic. This is in a measure substantiated by the fever attending trichinosis, where the question of ptomain poisoning has not yet been raised.