THE SOMITES OF THE CHICK 63 



it is an exceedingly thin and imperfect sheet, composed of small 

 irregular clusters of stellate cells or cell-like masses of protoplasm. 

 The portion of the mesoderm in front of the primitive streak, 

 which Rabl names the gastral mesoderm, is bisected by the noto- 

 chord. Small isolated cavities appear in the larger clusters of 

 cells of the antero-lateral portion of the gastral mesoderm and, 

 gradually enlarging, unite with one another, forming on either 

 side the parietal or amnio-cardiac cavity. The portion of the 

 gastral mesoderm below the medullary plate and beside the noto- 

 chord is thicker than its lateral part, and the first intersegmental 

 cleft appears in this thick medial edge at a point nearly midway 

 between the anterior extremity of the embryo and the primitive 

 streak. This cleft, as is well known, is somewhat V-shaped, being- 

 prolonged on each side of the median line laterally and slightly 

 backward. Patterson ('07) and Miss Hubbard ('08) have proven 

 experimentally that this cleft is the most anterior of the entire 

 series and not, as Von Baer thought, the third from the head. 

 It forms the posterior boundary of the first segment, which, being 

 continuous anteriorly with the unsegmented mesoderm of the 

 head, is incomplete. Each somite of this segment produces ap- 

 proximately half as much muscle and mesenchyma as the following 

 somites. A second intersegmental fissure appears quickly and 

 cuts off the first pair of complete somites, those of the second seg- 

 ment. Each somite of this segment (fig. 1) is a very irregular 

 flattened mass of cytoplasm containing many nuclei, which are 

 rounded or oval and are without definite arrangement. There is 

 observable, however, a very faint indication of a division of the 

 somite into upper and lower layers. 



While the third intersegmental fissure is forming, there appears 

 a longitudinal constriction which lies parallel to, and a short dis- 

 tance from the notochord, and divides the gastral mesoderm into 

 a narrow medial zone, the somitic plate, and a broad lateral region, 

 the lateral plate. This constriction extends forward so as to form 

 the lateral boundary of the first and second somites and also ex- 

 tends backward some distance behind the third intersegmental 

 cleft. The somitic plate, like all other portions of the mesodermal 

 sheet, gradually becomes thicker toward the primitive streak. 



