THE SOMITES OF THE CHICK 



LEONARD W. WILLIAMS 

 From the Harvard Medical School, Boston 



NINETEEN FIGURES 



The first consistent account of the history of the somites of 

 the chick, in which was presented the much discussed theory of the 

 resegmentation of the vertebral column, was published by Remak 

 in 1855. He believed that the somites are originally hollow 

 cubical masses of cells which, as the medullary groove deepens, 

 become triangular prisms with dorsal, medial, ventral and end 

 walls. The medial ventral edge of each somite elongates and, 

 reaching the notochord, divides into two leaf-like processes which, 

 uniting with those of the opposite side, grow around the notochord 

 and form the tissue of the perichordal sheath or Wirbelkorper- 

 sdule. From the same edge, he believed, there grows into the 

 ca\ ity of the somite a mass of cells, the core Urwirbelkern, which 

 greatly reduces the cavity. The core of the somite soon fuses with 

 the neighboring walls with the exception of the dorsal wall. Each 

 somite is now divided into an epithelial or epithelioid upper wall 

 and a mesenchymal mass formed by the fusion of the core and walls 

 of the somite. Remak named the former the Riickentafel or 

 Muskelplatte. The latter, the sclerotome, he named the Wii^bel- 

 kernmasse. There soon appears a contrast between the anterior 

 and posterior parts of the sclerotome. The spinal nerve with its 

 ganglion and roots appears in the anterior portion of the sclero- 

 tome from which Remak believed that it arose. The posterior 

 part becomes condensed forming what Remak and others have 

 called the vertebral arch. The correctness of this interpretation 

 will be discussed later. Remak believed that toward the end 

 of the fourth da}'' this vertebral arch is pushed backward so 

 that its posterior edge is covered by the dorsal lamella or Rilcken- 



THE AMEUICAN JOURNAL OF ANATOMY, VOL. II, NO. 1. 



