Gh. XIV] ORGANS FROM THE MESODERM 155 



method by which the blood-vessels are formed lias not been 

 fully considered. The dorsal aorta is the first vessel to arise. 

 A series of isolated lacunae appear in the mesoderm along the 

 roof of the pharynx, and by opening into one another form 

 a pair of longitudinal vessels. Vessels next appear in the 

 first and second branchial arches. Similar vessels arise later 

 in the third and fourth branchial arches. In the hyoid and 

 mandibular arches the vessels appear, as we have seen, later 

 still. These branchial blood-vessels originate in part as iso- 

 lated lacume in the mesoderm, and in part as outgrowths of 

 already existing vessels. For instance, lacunar vessels appear 

 in the mesoderm of the gill-arches, two in each arch. One 

 of these is the efferent lacunar vessel, and later connects with 

 a corresponding diverticulum from the dorsal aorta, and the 

 other lacunar vessel is the afferent vessel of the same arch. 

 This latter vessel grows ventrally toward the diverticulum 

 from the truncus arteriosus and unites with it. 



The walls of the blood-vessels are formed directly from the 

 mesodermal cells around the lacunae. "The blood-corpuscles 

 are free cells that have been left in the lacuna-spaces, or more 

 usually are cells budded off at a later stage from the walls of 

 the vessels into their cavities." 1 At first the blood-corpuscles 

 are simply spherical cells containing yolk-granules. Only after 

 the embryo is hatched do many of the corpuscles begin to 

 acquire the shape and character of red blood-corpuscles. 



The Pronephros 



The excretory system of the young embryo is represented on 

 each side by the pronephros and the segmental duct. Whether 

 the pronephros and duct arise in part from an early ingrowth 

 of ectoderm or whether they develop in situ from the somatic 

 mesoderm is perhaps still open to doubt. Field ('91), who has 

 worked out most recently the development of the pronephros 

 and segmental duct in the frog, describes the organ as coming 

 entirely from the mesoderm. We shall follow closely Field's ac- 

 count. The pronephros appears at a stage when the medullary 



1 Marshall ('93). 



