THE SOMITES OF THE CHICK 59 



seemed to him to be the only evidence needed to prove the cor- 

 rectness of Remak's theory. The anterior half of one sclerotome 

 he believed fuses with the posterior half of the preceding sclero- 

 tome to form an intersegmental vertebra. 



This view was attacked by Corning ('97) who pointed out the 

 weakness of Von Ebner's position. He maintained rightly that 

 a midsegmental vertebral centrum does not exist and that the 

 sclerotome is not the primitive vertebral centrum of Remak 

 This criticism elicited a reply from Von Ebner ('92) which con- 

 tained the fundamental truth that the dense mass of tissue 

 forming the greater portion of the posterior part of the sclerotome 

 IS a composite structure. The ''primitive arches," i.e., the lateral 

 portions of the scleromeres, are merely segmental structures 

 which contain the anlagen of several diverse structures. 



A very different conception of the structure of the vertebral 

 column is that of Goette who in a series of papers, particularly 

 one m 1896, presents the theory that each segment contains pri- 

 marily the anlagen of one haemal and two neural arches, an inter- 

 centrum, and a centrum. In the tail of a well advanced embryo 

 of Lacerta, there is a gradual transition in the structure of the 

 neural arch. Anteriorly each half of the arch is a broad plate 

 of bone; in the middle of the tail it is divided by a deep vertical 

 groove, or by a narrow sht, into a large anterior and a small pos- 

 terior arm; and still farther back it is represented by two bars of 

 bone, of which the anterior is the larger. In adult lizards, how- 

 ever, the neural arch of every caudal vertebra is an undivided 

 broad plate. The transverse processes of the caudal vertebrae 

 of Lacerta, and also of certain mammals, show a similar tendency 

 to divide into anterior and posterior portions. The dense tis- 

 sue in the middle of each segment represents the intercentrum and 

 the loose tissue between every two intercentra and between the 

 right and left intersegmental arteries forms the primary centrum. 

 The bases of the neural arches broaden and fuse with the primary 

 centrum, forming the secondary centrum. The haemal arch ex- 

 tends from below the intercentrum backward and downward into 

 the intermyotomic septum. It may fuse with the posterior end 

 of the preceding vertebra, as it does in Lacerta, or with the anterior 



