CHAPTER THIRTEEN 



Tlie Mesodermal Derivatives 



The Epimere (Segmental or Vertebral The Reproductive System 



Plate) The Gonoducts 



The Somites The Gonads 



The Vertebral Column The Hypomere (Lateral Plate Meso- 



The Skull (Neurocranium of the derm) 



Adult) The Coelom (Splanchnocoel) 



The Visceral Arches The Heart 



The Appendicular Skeleton The Circulatory System 



The Mesomere (Intermediate Cell The Arterial System 



Mass) The Venous System 



The Pronephros or Head Kidney The Lymphatic System 



The Mesonephros or Wolffian Body The Spleen 



The Adrenal Glands 



The Epimere (Segmental or Vertebral Plate) 



The Somites. 



These dorsally located blocks of metamerically arranged meso- 

 derm are differentiated from the anterior to the posterior. The first 

 ones to be completed are located just posterior to the auditory cap- 

 sule. A total of 13 pairs is formed from the head to the base of the 

 tail. In the tail of the 6 mm. tadpole there may develop as many as 

 32 pairs of these transient somites, making a total of 45 pairs by the 

 6 mm. stage. The two most anterior or occipital pairs of somites 

 become indistinguishable and the larval tail somites are lost during 

 the process of metamorphosis. This leaves a total of 11 pairs of 

 somites from which develop most of the striated muscles and the 

 primary skeleton of the body. In the head and pharyngeal region the 

 mesoderm is in the form of loose mesenchyme. 



Each somite develops in a characteristic manner, having an outer 

 thin layer of cells known as the dermatome or cutis plate and a mesial 

 mass of cells known as the myotome or muscle segment. Between 

 these is a cavity, the myocoel, which becomes elongated dorso-ven- 

 trally as the cutis plate extends to the body wall and appendages. 

 Originally this myocoel is continuous with the splanchnocoel or 

 body cavity. The myocoel eventually becomes obliterated. From the 



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