86 LEONARD \V. WILLIAMS 



bral arches. Froriep shows that the primitive vertebral centrum 

 soon extends around the notochord and differentiates into the 

 perichordale Faserring and the cartihi^inous hypochordale Spange. 

 The former takes part in the formation of the intervertebral 

 joint or in mammals forms the intervertebral disc — the latter 

 is tlie intercentrum (Mannich, '02) or the haemal arch (Schauins- 

 land, '05). Froriep further shows that the vertebral centra are 

 formed from the loose tissue of the perichordal sheath between 

 the successive midsegmental condensations. I believe it will be 

 found that in the chick, as in the pig, cartilage arises from rela- 

 tiveh' loose, not from dense mesenchyma, or that when it does 

 arise in dense tissue it does so only after a previous loosening 

 up of the dense tissue. This is certainly true of the centrum and 

 intercentrum. We ought, I believe, to regard the scleromere as a 

 composite structure, not as a morphological or structural unit. 

 It is unwise, therefore, to try to homologize the sclerotomic 

 columns or halves or the scleromere with the vertebra or with any 

 of its elements. 



THE TWENTY-FTFTH SOMITE 



The twenty-fifth segment (fig. 13) at the time of its formation 

 is in a more advanced state of development than the second or the 

 tenth when they first appear, and it passes with great rapidity 

 through the first steps of its later development. In an embryo 

 of twenty-seven segments (H. E. C. no. 1520) each somite of this 

 segment is already divided into the sclerotome and the dorsal 

 lamella, and both myotomic grooves have appeared. The upper 

 myotomic groove, however, is very shallow and is so indistinct 

 that it is not readily seen. The lower groove entirely encircles 

 the somite and neural crest cells are already moving down into 

 its deep anterior end. The newly formed sclerotome has fused 

 with the preceding and following sclerotomes, but not with the 

 opposite one. It is continuous laterally with the mesenchyma of 

 the nephrotome and through it with that of the lateral plate. The 

 aorta, the posterior cardinal vein and the connecting interseg- 



