THE DEVELOPMENT OF THE THOEACIC VERTEBRA IN 



MAN. 



BY 



CHARLES R. BARDEEN, 

 Professor of Anatomy, the University of Wisconsin, Madison, Wisconsin. 



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 liter- 

 ature on this subject up to 1897 has been reviewed by Gaupp, 97.* 

 Somewhat less attention has been devoted to the mammals. To Fro- 

 riep, 86, is due a valuable account of the development of the cervical 

 vertebrae in the cow, and to Weiss, 01, an important description of the 

 development of the thoracic and cervical vertebrae 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 meso- 

 blast, 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 over- 

 lapping periods: a membranous or blastemal, a chondrogenous, and an 

 osseogenous." 



'Among more recent papers may be mentioned, those of Baldus, 01; Hay, 

 97; Kapelkin, 00; Manner, 99; Mannich, 02; Ridewood, 01; and Schauins- 

 land, 03. 



^ In the text books the first of these periods is usually called the precartilage, 

 prochondral, or YorknorprA stage, but the condensed tissue from which 

 the skeletal parts are derived gives rise not only to cartilage but also to 

 perichondrium and to ligaments. Recognizing this fact, Schomburg, 00, has 

 American Jouknal of Anatomy.- — Vol. IV. 



