THE MESOBLAST. 



(2) Part of the inter-muscular connective tissue. 



(3) 

 (4) 



skeleton and 



surrounding 



connective 



The axial 



tissue. 

 The muscular and connective-tissue wall of the 



alimentary tract. 



(5) Part of the peritoneal epithelium. 



In the region of the head the mesoblast does not at first 

 become divided into somites ; but on the formation of the gill 



A. B. 



sp.c 



lit 



FIG. 23. SECTIONS THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY 



YOUNGER THAN 28 F. 



Figure A shews the separation of the body cavity from the pericardia! cavity by 

 a horizontal septum in which runs the ductus Cuvieri ; on the left side is seen the 

 narrow passage which remains connecting the two cavities. Fig. B through a 

 posterior part of the trunk shews the origin of the segmental tubes and of the primi- 

 tive ova. 



sp.c. spinal canal ; IV. white matter of spinal cord ; pr. commissure connecting 

 the posterior nerve-roots; ch. notochord ; jr. sub-notochordal rod ; an. aorta; sv. sinus 

 venosus ; cav. cardinal vein ; ht. heart ; //. body cavity ; pc. pericardia! cavity ; crs. 

 solid oesophagus; /. liver; mp. muscle-plate; ;/'. inner layer of muscle-plate; Vr. 

 rudiment of vertebral body ; st. segmental tube ; sd. segmental duct ; sp.v. spiral 

 valve ; v. subintestinal vein. 



clefts a division takes place, which is apparently equivalent to 

 the segmentation of the mesoblast in the trunk. This division 

 causes the body cavity of the head to be divided up into a series 



