HISTOGENESIS OF THE MESODERMAL TISSUES 



283 



alveoli and alveolar ducts of the lungs is converted into the flattened 

 respiratory epithelium. 



The development of the thymus and thyreoid glands, liver and pan- 

 creas may be found in Chapter VII. 



HISTOGENESIS OF THE MESODERMAL TISSUES 



The differentiation of the mesoderm has been described on p. =13 ff. 

 It gives rise to the mesodermal segments, intermediate cell masses, somatic 

 and splanchnic layers, all of which are epithelia, and to the diffuse mesen- 

 chyme. The somatic and splanchnic layers of the mesoderm form on their 

 coelomic surfaces a single layer of squamous cells, termed the mesothelium. 



Somatic mesoderm 

 Splanchnic 



Mesodermal segment 

 Central cells of segment 

 Ederolome 

 Ectoderm 



Intermediate 

 cell mass 



Urogenital ridge. 



Splanch 

 mesoder 



FIG. 290. Transverse section of a 4.5 mm. human embryo, showing the development of the 

 sclerotomes (Kollmann). X about 300. 



This is the covering layer of the pericardium, pleurae, peritoneum, mesen- 

 teries, serous layer of the viscera, and lining the of vaginal sac in the scro- 

 tum. From this mesothelium is derived the spleen and also the epithelia 

 of the genital glands and the Mullerian ducts. 



The intermediate cell masses, or nephrotomes, are the anlages of the 

 pronephros, mesonephros, metanephros, and their ducts (p. 196). 



The Sclerotomes and Mesenchyme. The cavities of the mesodermal 

 segments become rilled with diffuse, spindle-shaped cells, derived from 

 the adjacent walls; their median walls are next converted into similar 

 tissue and the whole migrates mesially towards the neural tube and 

 notochord, and eventually surrounds these structures (Figs. 290 and 323). 

 This diffuse tissue is mesenchyme (see p. 55), and that derived from a single 



