THE LATER DEVELOPMENT OF THE FROG 179 



some cases the smaller vessels seem to be preformed as short 

 solid strands of cells, which become rearranged to form the 

 walls of hollow tubes. 



At first these vessels, like the heart itself, are devoid of 

 cellular (corpuscular) elements. Some have described the 

 origin of blood corpuscles directly from the walls of the vessels, 

 but it seems doubtful whether such a process is at all common. 

 For the most part the blood corpuscles are formed from a large 

 group of blood islands, groups of cells occupying the ventral side 

 of the yolk-mass, between the liver diverticulum and the ventral 

 margin of the original blastoporal region (Fig. 63). 



The ventro-lateral surfaces of the endodermal yolk-mass, as 

 we have seen, give off the mesoderm by delamination, but in 

 this ventral region the superficial cells of the yolk-mass split 

 off irregularly in groups. These cell groups are the blood islands 

 (Brachet). While some of these cells are converted into the 

 walls of the veins of the yolk, they are mostly transformed into 

 red blood corpuscles, which thus enter the circulation by way 

 of these veins. The corpuscles enter the circulation in Iarva3 

 of about 5 mm., and for some time their origin from the yolk 

 region is indicated by their abundant yolk content; not until 

 after hatching do they assume the histological characteristics 

 of the definitive corpuscles. 



3. The Arterial System 



The earliest arteries to appear (about 4 mm.) are .the paired 

 lateral dorsal aorta, dorsal to the pharyngeal region. At first a 

 series of separate spaces or lacunae in the mesenchyme of the 

 head, these soon connect forming definite vessels extending 

 forward into the cranial region. Posterior to the pharynx these 

 vessels unite forming the median dorsal aorta, which then 

 extends to the posterior extremity of the embryo. 



The blood vessels of the visceral arches develop very early. 

 Those of all the branchial arches are essentially similar, while 

 the arteries of the hyoid and mandibular arches are consider- 

 ably modified from the branchial type and are largely vestigial 



