The Development of the Heart in Shad. 2S0 



cardiiim. The endocardium, which now covers a considerable area, 

 not only lies over (ventral to) the closing edges of the entodermal 

 pharynx, but partially covers the ventral surface of the future roof 

 as well (see Fig. 6 and 6F). The portion moyenne is no longer 

 descending in the region anterior to the mandibular pouch, for here 

 it has, apparently, been exhausted (see Fig. 6B). The descent area 

 on the left side (right in Fig. 6C) is situated toward the back of 

 the mandibular pouch, and on the right (left in Fig. 6D) is altogether 

 behind this structure; here again there is slight asymmetry. The 

 lateral plates, which are now much increased in width, are approach- 

 ing the mid-line ; the notch in the medial border of each foreshadows 

 the point at which the actual borders never meet, but at which they 

 are about to embrace the root of the ventral aorta. 



The lateral margins of the somital portion of the head mesoderm 

 are, at this stage, spreading ventrally around the lateral margins 

 of the entodermal pharynx in the region between the mandibular and 

 hyo-branchial pouches (see Figs. 6C and 6D). The cells from the 

 somital mesoderm, which now partially embrace the gut ventrally, 

 will foiTti the muscle and supporting framework of the hyoid arch. 

 There is no difficulty noiv in distinguishing the head mesoderm proper 

 from the undescended portion moyenne, with which it is, in many 

 places, in close contact. The cells of the portion moyenne (like the 

 endocardial cells ventral to the entodermal pharynx) are becoming 

 plainly endothelial; they differ from the other cells of mesodermal 

 origin in that the nuclei appear, in transverse sections of the embryo, 

 to be small and rather flat and to be surrounded by a comparatively 

 large amount of cytoplasm. 



At a stage of 32 somites (Fig. 7, three hours later than the 

 preceding stage^^) the medial margins of the lateral plates have 

 met and blended throughout the anterior three-fourths of the peri- 

 cardial region except at one place foreshadowed in the preceding 

 stage. The medial margins of the lateral plates, where they fail to 



''In my stage of 30 somites (exactly intermediate in time between tiie 

 stage of Fig. G and that of Fig. 7) the conditions in the heart-region are 

 practically indistinguishable from those found at 32 somites (Fig. 7) ; the 

 later stage has been used for reconstruction on account of its superior preserva- 

 tion. 



