424 F. H. Krecker, 



pace with the enlarged diameter of tlie coelom in this region, elongate 

 both by the division and the attenuation of their constituent cells and 

 pari passu with this elongation the columns become separated from 

 each other so that the individual septa can be clearly distingiiished 

 (Fig. 14). While this elongation and Separation is in progress the 

 septal muscles are also in process of formation. Along the edges of the 

 columns the cytoplasm of the cells loses its granulär appearance, be- 

 comes more or less homogeneous and has a stronger affinity for eosin. 

 This change gradually extends to other parts of the septum until the 

 muscular tissue has been formed along its entire length. The descrip- 

 tion just given applies to the formation of septa in the ventral half or 

 two thirds of the coelom. On the dorsal side it is somewhat different, 

 although the material concerned is largely of neoblastic origin as will 

 be set forth in the discussion of the longitudinal musculature. Ran- 

 DOLPH (92) termed the cells in the dorsal region of the coelom the 

 "dorsal mesoblast". These cells are applied to the dorsal surface of the 

 dorsal blood vessel and also to the surface of the body wall in this 

 region. From both the blood vessel and the body wall the cells send out 

 processes which either meet each other or eise, when the space between 

 the body wall and blood vessel is not wide, a single process extends 

 entirely across (Fig. 13). 



Probably the very first organs to assume definite form are the 

 longitudinal muscles for, while there is as yet an apparently unorganized 

 group of cells in the coelom one can distinctly see between the base 

 of the neoblasts and the body wall a layer of longitudinal muscle which 

 is extremely thin near the posterior limit of the neoblasts but which 

 becomes thicker as the point of union with the old musculature is 

 approached. These muscles are derived from the comparatively small 

 neoblasts which lie close to the body wall (Fig. 14). The cells destined 

 to form the muscle, except for their size and position, do not at first 

 differ perceptibly from their neighbors. Then they gradually elongate 

 and about the periphery the granulär cytoplasm assumes the appear- 

 ance characteristic of smooth muscle. During this process the nucleus 

 also changes; it becomes smaller, elongates in the longiaxis of the cell, 

 loses the prominent nucleolus and also has a blue color after treatment 

 with hematoxylin and eosin in place of the purplish hue of the neo- 

 blasts. The posterior neoblasts give rise to the most posterior of the 

 muscle cells but the more anterior neoblasts form the greater part of 

 the muscle. As noted by other workers the formation of the longi- 

 tudinal muscles does not occur simultaneously about all parts of the 



