extent. Fig. 34 shows similar nodules in the purely acinous part of 103 tumour to which reference has been made above. 0 In this case such nodules are frequent throughout, but principally in connection with tubules of which the epithelium resembles more closely that of the ducts of normal mamma rather than of the secreting acini. In this respect they recall the similar nodules frequently met with in adenomata of the mamma of the human subject and generally described as cholesteatomata. Henke regards these formations as closely allied to keratinisation, and in the mouse tumour under discussion eleidin granules. and prickle-cells have been demonstrated. 32 These observations have an important bearing on some of the questions raised by the co-existence of acinous and keratinised areas in our transplantable squamous-cell carcinoma () and by C. Lewin in his account of a transplantable mammary carcinoma of the rat. This tumour of Lewin presented the structure of an adeno-carcinoma in the primary animal. It was only after propagation through several generations (3rd) that wide-spread keratinisation was observed. Lewin held that the keratinising tumour had developed by infection from the mammary carcinoma of the overlying epidermis covering a subcutaneous nodule. As an alternative he suggested that metaplasia had supervened, transforming the acinous tumour into a squamous-celled carcinoma. It will be recollected that a transplantable squamous-celled carcinoma in which the connection with the skin of the axilla could be demonstrated as described on p. 81, presented acinous areas in addition to the main mass which was keratinised or alveolar. Whether the primary tumour in Lewin's case had a correspondingly varying structure it is impossible to say, as histological examination was very incomplete. Whether keratinised elements were present in the material used for his primary transplantation or not, seems immaterial in the light of our experience with a transplantable squamous-celled carcinoma which remains alveolar for long periods and at intervals presents wide-spread keratinisation in the daughter-tumours. In addition quite recently acinus-like lumina have appeared in the alveoli of several distinct strains. We may conclude with a high degree of probability that we are in this and in Lewin's case dealing with several growth-forms of a somewhat polymorphous parenchyma, as already suggested for the chrondro-osteoidsarcoma at pp. 78-79. To speak of metaplasia or anaplasia in this connection seems inappropriate unless we use these terms with the qualification that they do not necessarily imply any permanent nonreversible alteration in cell-characters. We prefer, therefore, to use the expression "growth-form" for these histological variations, and regard them as indications of the close association of the mammary apparatus with the skin from which it develops. In addition we must recollect that the ampulla which receives the terminal portions of the mammary ducts, is also lined by stratified squamous epithelium and may be regarded as a secondary invagination of the covering epithelium. Therefore, should the cells of a new growth have taken their origin from a part of the gland close to the nipple (and this is frequently the case, see fig. 20), variations in either direction are only to be expected. |