CH/ETOPODA. 



323 



These rows of cells form the commencement of the mesoblastic 

 bands, and in the succeeding stages they extend one on each 

 side of the body (fig. 141 B, ms) till they reach the sides of the 

 mouth. Their forward growth takes place mainly at the 

 expense of the superjacent epiblast cells, but the two mesoblasts 



FIG. 141. THREE SECTIONS ILLUSTRATING THE DEVELOPMENT OF LUMBRICUS 



TRAPEZOIDES. (After Kleinenberg.) 

 ms. mesoblastic band ; m'. mesoblast ; al. archenteron ; pp. body cavity. 



A. Horizontal and longitudinal section of an embryo which is dividing into two 

 embryos at the gastrula stage. It shews the mesoblasts and the mesoblastic bands 

 proceeding from them. 



B. Transverse section shewing the two widely-separated mesoblastic bands. 



C. Transverse section at a later stage shewing the mesoblastic bands which have 

 approached the ventral line and developed a body cavity//. 



at their hinder extremities probably assist in their growth. 

 Each mesoblastic band is at first composed of only a single row 

 of cells, but soon becomes thicker, first of all in front, and 

 becomes composed of two, three or more rows of cells abreast. 

 From the above it is clear that the mesoblastic bands have, in 

 L. trapezoides at any rate, in a large measure an epiblastic 

 origin. 



At first the two bands end in front at the sides of the mouth, 

 but subsequently tbeir front ends grow dorsalwards at the 



21 2 



