Growth of cells cultivated in vitro can be estimated by various methods devised by many research workers in the field of tissue culture studies. Generally speaking, those estimating methods may be divided into some lines, that is, areal measuring of cell colonies in cultures with solid phase of growth, cell number counting in the state of cell suspension, and many other physicochemical methods.
Areal measuring method originally adopted by Carrel on coverslip or flask cultures supported by plasma clot, was later modified by many workers from several directions, following the improvement of tissue culture technique itself. While coverslip or flask culture technique had been thus improved, Gey devised another method of tissue culture, so-called roller-tube technique, on the basis of Carrel's suggestion, and he and his associates succeeded to separate and establish a cell strain of rat origin under continuous in vitro cultures using this roller-tube technique. This strain, which has undergone the later change in biological character to become malignant, grows easily on the glass wall of the tube without any substrate, forming almost monolayer cell colonies. Ehrmann and Gey used this characteristic strain to evaluate nutritional substances in vitro, and obtained more precise results than those in the past.
Meanwhile, Earle and co-workers established, after many years' studies on tissue culture, a more accurate quantitative method of growth estimation by cell nuclei counting, depending on their own device of replicate suspension cultures. This counting method can represent the growth of cells more exactly than areal measuring method, though it is more or less complicated in handling or procedure.
The present authors have been studying on tissue culture from several viewpoints. Our special interest is in cultivating malignant tumor cells to pursue their biological characters in rather broad meaning.
In the present paper, will be reported the results of several experiments concerning fundamental culture conditions of normal tissue cells including embryonic heart of chick, mouse, and human origins. Roller-tube technique and areal growth measuring of cell colonies were used throughout experiments. Some methodological points will be discussed.

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The infection is a phenomenon which results from complex combinations of the factors both of host and parasite. This suggests that susceptibility should be considered from the viewpoint of host-parasite-relationship. If the condition of parasite, therefore, is kept as constant as possible, the susceptibility could be compared between different species by the reaction of host. While the study of susceptibility should be attempted from various standpoints, histological observations of the responses of the host would certainly be of some help to the analysis of susceptibility of host. Among the diverse responses observed in hosts when parasites invade them, those which develop into granuloma were taken as experimental materials. In the present study, the attempt was made with tuberculous granuloma as one example.
Much study has been done with experimental tuberculosis, while, few reports concerning a comparative observations on several host-parasite-combinations have been announced so far. Corper and Lurie reported their extensive studies which attempted to compare susceptibilities of some species of animals and those of their organs. Specifically their observations concerned the distribution of tubercle bacilli and the localization of tuberculous lesions, the ability of the cells composing tubercles to destroy parasites, and the differences of the oxygen supply in different tissues as a factor which influenced the degree of multiplication of the bacilli. Vorwald, through the observation of histological changes of lung, among various species of animals, researched on the modification of susceptibility as the results from the differences between native resistance and the ability to acquire resistance. However, they did not view the problem about the process of tubercle formation and its morphological characteristics. Lately Ratcliffe et al observed histologically the responses of hosts to the infection with tubercle bacilli during the period in which the nature of their responses shifted from the homogeneous phase to the heterogeneous phase: about susceptibility of the hosts, they concluded that there was no difference in native or inherent resistance between the hosts, though the difference of ability to acquire resistance existed among the species of animals.
In the present work, the observation has been limited to the tuberculous granuloma: Observed in several species of animals were the period of the formation of epithelioid cell tubercle, its morphological characteristics and the distribution as well as the behavior of the tubercle bacilli histologically demonstrated.
So far, the term “susceptibility” seems to have been used to denote somewhat different matters by each author. As its nature has not been explicitly defined, it is not an easy task to set an exact definition for it. In order to avoid the confusion, therefore, it must be first understood that in this paper the term is used to signify the reverse of resistance, without regard to either natural or acquired, and that such idea as “the factor of susceptibility” mentioned by Ando is not specifically considered.

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