Charles E. Bardeen 165 



sclerotome. From here the couJciisation extends dorsally between the 

 medial surface of the posterior half of the corresponding myotome and 

 spinal ganglion and gives rise to a dorsal, or neural, process (Figs. 5 and 

 6, Plate 2, N. Pr.). Kt the same time it proceeds ventrally along the distal 

 margin of the corresponding myotome and gives rise to a ventral or 

 costal process (Figs. 5 and 6, C.Pr.); and medially toward the chorda 

 dorsalis, giving rise to a process which joins about the chorda dorsalis 

 with a similar one, from the other side of the segment (Figs. 5 and 6, 

 Disk). These median processes by their fusion form what has been 

 termed by Weiss, oi, a "horizontal plate." "Primitive dish" seems to 

 me perhaps a better term. 



The whole mass of condensed tissue which gives rise to the primitive 

 dorsal, ventral and median processes has received various designations, 

 of which that given by Froriep, 83, " primitive vertebral arch " seems to 

 be the most widely accepted. Since, however, it represents much more 

 than a vertebral semi-arch, I have previously, 99, suggested for it the 

 term " scleromere." * 



Figs. 8, 9 and 10, Plate III, represent wax-plate reconstructions of 

 several scleromeres from the thoracic region of Embryo II, length 7 mm. 

 The outlines of the condensed tissue are not so sharp in nature as it is 

 necessary to make them in a model of this kind. It is believed, however, 

 that the general form relations are here fairly accurately shown. In 

 Fig. A, Plate II, of the article by Bardeen and Lewis are shown the rela- 

 tions of the scleromeres to other structures. 



During the period of differentiation of the scleromeres the myotomes 

 undergo a rapid development. The median surface of each myotome 

 gradually protrudes opposite the fissure of v. Ebner. The dorsal and 

 ventral processes of each scleromere are then slowly forced into the inter- 

 val between the belly of the myotome to which it belongs and the one 

 next posterior, and thus finally they come to occupy an intersegmental 

 position. It is not, however, correct to call the early processes of the 

 scleromeres '' myosepta" as some text-book writers have done. Fig. 4 

 shows this. 



* By the fissure of v. Ebner each sclerotome is divided into two portions, 

 of which the posterior in the higher vertebrates plays the chief role in verte- 

 bral differentiation. " Scleromere " therefore seems an appropriate designa- 

 tion for the condensed sclerogenous tissue of this half-segment. Goette has 

 recently, 97, brought forward evidence in favor of the view that primarily in 

 the digitates there were two vertebrae to each body-segment. In the higher 

 vertebrates, during embryonic development, the posterior skeletal area of 

 each body-segment alone develops freely. The anterior area becomes fused 

 with the scleromere in front. 



