284 



HISTOGENESIS 



lar ducts of the lungs is converted into the flattened respiratory epitheKum. The 

 development of the thymus and thyreoid glands, liver and pancreas has been 

 described in Chapter VII. 



Histogenesis of the Mesodermal Tissues 



The differentiation of the mesoderm has been described on p. 51, Fig. 53. 

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

 splanchnic layers, all of which are epithelia, and to the diffuse mesenchyme. The 

 somatic and splanchnic layers of the mesoderm form on their coelomic surfaces a 

 single layer of squamous cells termed the mesothelium. This is the covering layer 



Mesodermal segment' 

 Central cells of segment 



Sclerotome- 



Ectoderm- 



Intermediate 

 cell mass 



Urogenital ridge. 



Splanchnic 

 mesoderm 



Somatic mesoderm 



Splanchnic 

 — mesoderm 



, Fig. 290. — Transverse section of a 4.5 mm. human embryo showing the development of the sclerotomes 



(Kolhnann). X about 300. 



of the pericardium, pleurae, peritoneum, mesenteries, serous layer of the viscera, 

 and lining of the vaginal sac in the scrotum. From this mesothelium is derived 

 the spleen and also the epithelia of the genital glands and the Miillerian ducts. 



The intermediate cell masses or nephrotomes are the anlages of the pro- 

 nephros, mesonephros, metanephros, and their ducts (p. 195). 



The Sclerotomes and Mesenchyme.— The cavities of the mesodermal seg- 

 ments become filled 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 sur- 

 rounds these structures (Figs. 290 and 323) . This diffuse tissue is mesenchytne (see 

 p. 53), and that derived from a single mesodermal segment constitutes a sclerotome. 



