04 LEONARD \V. WILLIAMS 



In embryos of three segments, the second somite (fig. 2) is 

 quadrangular and is considerably thicker than before. It is now 

 distinctly divided into upper and lower layers, which are con- 

 tinuous with one another at the edges of the somite. The longest 

 axes of the oval nuclei radiate from the center of the somite. 

 Its surface is still very irregular. 



New somites are constantly forming, and each differs more or 

 less from the others, as will be seen in figs. 10 to 14, which represent 

 transverse sections of several newly formed somites. It seems 

 wisest, therefore, to study the development of the somites of a sin- 

 gle segment, the second, and then to compare these with the others. 



In embryos of six segments (fig. 3) the edges of the medullary 

 plate are considerably elevated above the second segment and 

 each of the somites of this segment has become an irregular tri- 

 angular prism with dorso-medial, dorso-lateral, lower, and anterior 

 and posterior surfaces. The first two of these arise from the upper 

 surface of the earlier somite. The dorso-medial or medial surface 

 is concave and is molded against the convex lower surface of the 

 medullary plate. The dorso-lateral (which will soon become the 

 upper) surface is quite irregular. The lower surface is convex. 

 The medial edge of the somite is almost in contact with the noto- 

 chord and its anterior end is prolonged forward so that the anterior 

 end or wall of the somite slopes from above downward and for- 

 ward. The somite now contains a flattened core consisting of a 

 few rounded nuclei and a small amount of cytoplasm. 



The coelom expands rapidly and has now divided the greater 

 part of the lateral plate into an upper or parietal and a lower or 

 visceral layer. It appears also in the thicker medial edge of the 

 lateral plate, forming a slight expansion (C) and finally sends a 

 small tortuous prolongation through the constricted area or stalk 

 which connects the second somite and the lateral plate. Thus a 

 small prolongation of the coelom enters the somite and separates 

 the dorsal wall of the somite from the core. It is now clear that 

 each somite of the second segment for a time has no central cavity 

 and is without a core. This fact was overlooked by Remak 

 who believed that each newly formed somite has a central cavity, 

 and by His who states that from the first the somite has a core. 



