ANNELIDA 



339 



uraen, and lies as a cell alar cord in the ventral median 

 of the embryo. 



On each side of the nerve cord lies a ribbon-like cord of 

 cells, the mesodermal band. The two mesodermal bands are 

 united to each other by a median part. They have arisen 

 from the common ento-mesodermal mass, the origin of which 

 we have previously traced, by the separation of a ventral 

 layer, the mesoderm, from the dorsal layer, the entoderm. 

 A segmentation, like that 

 in the ventral cord, also 

 makes its appearance in 

 the mesodermal bands, 

 which separate into the 

 primitive segments. The 

 processes thus effected, as 



ell as the formation of 



e body cavity and the 

 septa, take place in a 

 manner similar to that 

 described for the other 

 Annelida. The internal 

 segmentation is late in 

 finding expression on the 

 exterior of the body, and 

 is suppressed in its an- 

 terior and posterior parts, 

 where the primitive seg- 

 ments for the present 

 acquire no cavities, and 

 therefore remain in an 

 embryonic condition. 

 Each segment exhibits 

 externally a division into 

 a broader and a narrower portion (Fig. 157 E). The former 

 corresponds to a ganglion, the latter to a septum. In front 

 of the anterior end of the ganglionic chain lies a part of the 

 mesoderm, which forms the head cavity ; but regarding this, 

 Salensky could not determine whether it Hkewise arose from 

 the mesodermal bands. 



Fig. 157.—^ to E, embryos of Bratichio- 

 bdella in vadous stages (after Salenskt). 

 ect, ectoderm ; gr, pit in the entoderm on 

 the dorsal side; ma, macromeres ; m, mouth- 

 opening ; n, neural groove j «, sucker. 



