204 PROFESSOR MARSHALL AND EDWARD J. ELES. 



cells which form the walls of the vessels. They are at first spherical, 

 nucleated, and richly studded with yolk granules. 



7. General Considerations. 



The above description raises several questions of great interest, 

 which it will be well to consider at once. The points of most 

 importance appear to be the following : — 



i. The heart is from the first in connection with the veins of the 

 yolk mass ; it is, indeed, formed by the union of these veins, and its 

 endothelial lining is derived, in part at any rate, from the yolk cells. 



ii. The heart has at first, and for some time, no connection with 

 the branchial vessels. At 4| mm. the heart is considerably twisted 

 on itself, and all its main divisions may already be recognised, the 

 truncus arteriosus being, perhaps, the most clearly differentiated. 

 Vessels are already present in the hyoid and the first three branchial 

 arches, and in the first and second branchial arches have attained 

 considerable development. None of these vessels, however, have any 

 connection with the heart. 



iii. The dorsal aortas form very early in the roof of the pharynx, 

 and soon acquire connection with the efferent branchial vessels : 

 posteriorly the aortas extend a very short distance behind the head. 



iv. In the visceral arches the efferent blood vessels are the first to 

 appear. At 4|- mm. these are the only vessels present in the hyoid 

 and the third branchial arches, where they have the form of irregular 

 lacunar spaces, apparently intercellular in origin, and having as yet 

 no connection with any other vessels. In the first and second 

 branchial arches the efferent vessels are of much larger size, and 

 extend almost the whole length of the arches; they are widest 

 opposite the external gills, and are prolonged ventralwards almost to 

 the truncus arteriosus, while dorsally they open into the aorta. The 

 greater part of the length of the vessel is still lacunar, but the dorsal 

 part, in communication with the aorta, is a well-formed tubular vessel, 

 with walls formed of flattened epithelial cells. This distinction is 

 important, because examination of earlier stages shows that this part 

 of the vessel is formed as an outgrowth from the aorta towards the 

 lacunar part of the efferent vessel, and not by extension of the 

 lacunar vessel to meet the aorta. 



The arrangement calls to mind the continuity between the vascular 



