vi ORIGIN OF THE HEART AND VESSELS 363 



By the twenty-seven segment stage the anterior limbs of the 

 subintestinal strand have become definite (vitelline) veins with 

 well-defined lumen filled with fluid in which spherical young 

 corpuscles float freely. The large flat cells forming the wall are 

 probably simply the modified superficial cells of the strand though 

 Mollier thinks these may be reinforced by additional mesoderm cells 

 from without. The vitelline veins are continued in front into the 

 posterior venous limbs of the heart and the heart itself is seen in 

 transverse sections (Fig. 172, E) to be now completely enclosed in 

 myocardium, the inner wall of the pericardiac space having become 

 moulded right over its dorsal side. Where the two sheets of mesoderm, 

 one from each side, have met dorsal to the heart there still persists a 

 septum — the dorsal mesocardium (Fig. 172, E, d.mc) which serves 

 to sling up the heart to the ventral side of the foregut. 



The dorsal aorta is at this stage particularly instructive. 

 Posteriorly it is represented by scattered cells, lateral in position, 

 thus betraying their lateral origin. Further forwards these have 

 approached the mesial plane and form a pair of cellular strands. 

 Further forwards still — in the region of the first eight segments — 

 these have become still more nearly mesial in position and over 

 part of their extent have undergone actual fusion to form the 

 unpaired aortic rudiment. 



About this stage the dorsal aortic rudiment is connected up to 

 the vitelline network by a series of segmentally arranged vessels 

 (segments 5-17) which had made their appearance about the twenty- 

 segment stage as segmentally arranged strands of cells. 



The rudiments of the ducts of Cuvier make their appearance 

 even earlier than that of the dorsal aorta, in the form of cells derived 

 according to Mollier from the somatic mesoderm at the cranial side 

 of the pronephros. These rudiments develop extensions in a 

 headward and in a tailward direction to form the cardinal veins. 

 The vessels of the head region develop in situ from the mesenchyme 

 and the same may probably be said of the smaller vessels generally. 



The Crossopterygian fish Polypterus (Graham Kerr, 1907) is, 

 apart from its generally archaic character, particularly suitable for 

 the study of the first beginnings of the vascular system owing to the 

 fact that the long axis of the embryonic body is straight, so that 

 horizontal as well as sagittal sections may be made passing through 

 practically the whole length of the dorsal aorta during its early 

 stages, when in its hinder portion it has not yet taken definite form. 



The first conspicuous stage in the development of the dorsal 

 aorta consists in the collecting together of irregular multinucleate 

 masses of yolky protoplasm in a row beneath the hypochord 

 (Fig. 174, A). Vacuolar spaces develop in these masses and fore- 

 shadow the aortic cavity. The masses of protoplasm become more 

 closely aggregated into a cylindrical shape while the vacuolar 

 spaces increase enormously in size and eventually flow together to 

 form the continuous aortic cavity. In the specimen figured in 



