80 EDWARD MCCRADY, JR. 



process which leads to the concrescence of the heart tubes 

 leads to the formation of a pharyngeal floor as seen in 

 figure 27. By the end of stage 24 this process has extended 

 the floor beneath the first branchial pouches (fig. 31), but the 

 second branchial pouches are still over the empty yolk sac. 

 During stage 25 the anterior intestinal portal moves caudad 

 beneath them as the ventricles of the hemicardia draw near 

 each other. The further migration of the portal caudad and 

 some parallel changes in the shape and differentiation of the 

 pharynx will be discussed in chapter XII in connection with 

 the lung pouches. 



Lung anlagen. Though the more familiar steps in the 

 development of the lungs come after stage 27, the earliest 

 primordia are distinguishable in late specimens of stage 24 

 and in all specimens of stage 25 (McCrady, '36). These pri- 

 mordia need special comment as they are different from any 

 that have hitherto been described. 



At stage 24 the endoderm in the cervical region shows on 

 its ventral surface three longitudinal depressions: a medial 

 notochordal groove, and two lateral branchial furrows. Each 

 branchial furrow deepens locally at two points so as to ex- 

 tend through the flat body wall to the ectoderm. The points 

 of contact between endoderm and ectoderm thus estab- 

 lished have already been pointed out as the closing plates of 

 the first and second branchial pouches. Posterior to them the 

 furrow becomes shallower until it gradually disappears near 

 the anterior limit of the pronephric anlagen. No such furrow 

 ever develops in the endoderm of the thoracic, lumbar, and 

 sacral regions. 



At the level where the sections show the anterior end of 

 the allantoic vein protruding through the somatopleure to- 

 ward the vitelline vein, the pleuro-pericardial cavity is in- 

 serted between the lateral half of the branchial furrow and 

 the ectoderm, and the splanchnopleuric mesoderm which thus 

 comes into contact with the endodermal wall of the groove 

 is conspicuously thickened. This thickening which is a center 

 of rapid proliferation, is the mesodermal blastema of the lung. 



