THE MESOBLAST. 



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



(3) The axial skeleton and surrounding connective 



tissue. 



(4) 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 



Ch- 



ill: 



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 pericardial 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 ; W. white matter of spinal cord ; pr. commissure connecting 

 the posterior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; sv. sinus 

 venosus ; cav. cardinal vein ; ht. heart ; //. body cavity ; pc. pericardial cavity ; <xs. 

 solid oesophagus ; /. liver ; mp. muscle-plate ; mp'. 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 



