62 EDWARD MCCRADY, JR. 



gradually diverging laterad to pass from beneath the medul- 

 lary plate at about the level of the fourth somite. Beyond 

 this point they give off a few small intersomitic twigs, from 

 which branches pass to the limb ridge, and finally they break 

 up into a complicated plexus leading ventrally and laterally 

 into the area vasculosa. 



In connection with the hearts a most unexpected detail is 

 represented in the totomount drawing (fig. 20) between the 

 levels indicated by the arrows D and E. Here may be seen a 

 large vessel branching from what appears to be the heart 

 tube medially toward a slight mesenchymal opacity at the 

 edge of the medullary plate. This vessel turns out to be the 

 primordium of the second aortic arch, and its point of junction 

 with the lateral tube marks the anterior extremity of the 

 heart tube proper. The mesenchymal condensation is the 

 hyoid mesenchyme. In the late specimens of stage 24 the 

 head region is rounding up so that the vessel, which ran 

 medially at first, comes to run dorsally into the hyoid arch. At 

 this time there is no corresponding dorsal twig from the aorta, 

 but such a twig develops in stage 25. Then the capillary 

 plexus and every stage in the differentiation of the arch can 

 be followed in stages 26 and 27. 



Four veins appear during stage 24. The first, the vitelline 

 (or omphalomesenteric), is the vein of the yolk sac. This 

 will be by far the most important vein in the embryo until 

 just before birth. It grows out as a capillary plexus from the 

 posterior end of the heart tube, which meets a similar plexus 

 arising in the area vasculosa and growing toward the embryo. 



The next two veins, the vena capitis medialis (or primary 

 head vein) and the allantoic (or umbilical) arise simultane- 

 ously. Many small vessels which appear to be remnants of 

 the same primitive capillary plexus from which the aortae 



Fig. 20 Totomount, sections, and reconstructions of stage 24. Totomount 

 Edinger apparatus drawing from 17176 with arrows to indicate levels of sec- 

 tional views. The level marked with the asterisk is shown in sectional view in 

 I and J. All sectional views and reconstructions are from 17153. M indicates 

 the actual size of vesicle, embryo, and area vasculosa. 



