i62o A MANUAL OF ANATOMY 



formed. The mesoderm of each lateral plate becomes thereby dis- 

 posed in two lamellae — outer or parietal, and inner or visceral. The 

 parietal layer is called the somatic mesoderm, and the visceral layer 

 is known as the splanchnic mesoderm. The somatic mesoderm 

 joins the ectoderm, and so the somatopleure is formed. The 

 splanchnic mesoderm joins the entoderm, and the two constitute the 

 splanchnopleure. 



The cavity formed within each lateral plate, and dividing that 

 plate into its somatic and splanchnic lamellae, is called the coelom 

 or primitive body-cavity. It is also known as the pletiro-peritoneat 

 cavity. It subsequently gives rise to the pleural, pericardial, and 

 peritoneal serous cavities, and also to the cavity of each tunica 



Somatopleure 



Splanchnopleure 

 Ccelum 



Entoderm 



-Neural Tube 

 Notochord 



Fig. 672. — Transverse Section of Body of an Amphioxus 

 (modified after Hatschek). 



vaginalis in connection with the testes. The mesodermic cells 

 which line the coelom represent mesothellum, subsequently known 

 as endothelium. 



The paraxial mesoderm, as stated, is disposed as two longitudinal 

 columns of thickened mesoderm, one being situated on either side 

 of the neural canal and notochord. Each of these columns under- 

 goes segmentation, and is broken up into a number of primitive 

 segments, called mesodermic somites, cervical, thoracic, lumbar,, 

 sacral, and coccygeal respectively. The process of segmentation is 

 preceded by the appearance of a series of transverse furrows along 

 each column. In this manner there is formed, in each longi- 

 tudinal column of paraxial mesoderm, a series of cubical somites. 

 There are thus two parallel rows of mesodermic somites, one right 

 and one left, situated one on either side of the neural canal and noto- 

 chord . 



The somites on either side become gradually separated from the 

 contiguous lateral plate of mesoderm by a longitudinal groove. 

 Along this groove a row of cells appears, known as the intermediate 

 cell-mass by which the somites are for some time connected with 



