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WITH 8 PLATES AND 27 TEXT FIWBES.The purpose of the following paper is a description of various typical stages in the development of the back, the limbs, and the body-wall in man. The work is based primarily upon reconstructions, according to the method of Born,' of parts of five human embryos; it has been extended and controlled by a study of the external form and of serial sections of several other human embryos. Dr. Lewis has devoted special study to the formation of the arm, Dr. Bardeen to that of the leg, the bodp-wall and the back. I n the accompanying table a list is given of the embryos utilized.Those marked with an asterisk have been reconstructed.We shall consider the early stages in the development of the limbs, the body-wall and the back, first, from the point of view of the external form and, secondly, from that of internal structural differentiation.

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WITH 7 PLATES.There is a somewhat extensive literature dealing with the development of the spinal column in various vertebrates. The chief stages in its differentiation are fairly well determined. Special attention has been given to the early development in the lower vertebrates. The recent literature on this subject up to 1897 h q been reviewed by Gaupp, 97.l Somewhat less attention hm been devoted to the mammals. To Proriep, 86, is due a valuable account of the development of the cervical vertebm in the cow, and to Weiss, 01, an important description of the development of the thoracic and cervical vertebrs in the white rat.We shall not attempt to enter here into a description of the early stages of differentiation in the spinal axis; that is of the period covering the formation of the chorda dorsalis and of axial and peripheral mesotlast, the differentiation of primitive segments, and the origin of the axial mesenchyme. This period in the human embryo has been well treated by Kollmann, 91, and some account of it has previously been given in this JOURNAL (Bardeen and Lewis, 01). We shall therefore proceed at once to a consideration of vertebral differentiation in the axial mesenchyme.Vertebral development in the embryo may be divided into three overlapping periods: a membranous or blastem~al, a chondrogenous, and an osseogenous.'