HISTOGENESIS OF THE MESODERMAL TISSUES 



291 



stratified ciliated epithelium. The columnar epithelium of the alveoli and al- 

 veolar ducts of the lungs is converted into the flattened squamous respiratory 

 epithelium. 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. 61, Fig. 51. 

 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 



Mesodermal 



segment 



Central cells of 



Sclerotome 

 Ectoderm 



Intermediate 

 cell mass 



Urogenital ridge 



Splanchnic 

 mesoderm 



Somatic 

 mesoderm 



Splanchnic 

 mesoderm 



Fig. 282. — Transverse section of a 4.5 mm. embryo showing the development of the sclerotomes 



(Kollmann's Handatlas). 



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 

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

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

 also the epithelium of the genital glands and that of the Muellerian ducts. 



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

 nephros, mesonephros, metanephros, and their ducts (p. 203). 



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

 ments become filled with diffuse spindle-shaped cells, then their median walls are 

 converted into similar tissue which migrates mesially towards, and eventually 

 surrounds, the neural tube and notochord (Fig. 282). This diffuse tissue is 



