368 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



fluid. The myocardium develops as in the other forms already 

 mentioned. 



Origin of the Peripheral Blood-vessels in the Meroblastic 

 Vertebrates. — In those Vertebrates which have meroblastic eggs 

 the concentration of yolk in the highly modified ventral endoderm 

 accentuates the need of an efficient transport system by which this 

 food material may be taken up and conveyed to the yolkless and 

 actively developing parts of the embryo. In accordance with this 

 we find that such vertebrates show a precocious development of 

 a rich network of blood-vessels over the surface of the endoderm 

 or yolk and this vitelline network affords admirable material for 

 the study of the earliest stages in the development of peripheral 

 blood-vessels. 



It will be convenient to consider in some little detail the early 

 development of the vitelline network in the Fowl, material for the 

 study of which is easily obtainable and which further has been 

 worked out in detail by numerous investigators. Riickert has 

 furnished an excellent modern account (1906) and upon it the 

 following description is based : for fuller detail reference must be 

 made to the original. 



The first signs of blood-vessel formation make their appearance 

 extremely early, at a stage when the primitive streak is present but 

 hardly any trace, or only a small stump, of the so-called head-process 

 at its front end. The mesoderm has at this stage spread slightly 

 beyond the edge of the pellucid area, especially posteriorly. Round 

 its posterior edge the mesoderm assumes a mottled appearance owing 

 to the development in it of small cell condensations — the first trace 

 of the blood-islands as they were called by Pander. These blood- 

 islands are sometimes arranged in two separate series one on each 

 side but more usually they form a U-shaped arrangement parallel 

 and in close proximity to the posterior limit of the mesoderm. 



In a slightly later stage the mottled area containing blood- 

 islands — the vascular area — is of a U -shape, extending through 

 about the posterior half of the extent of the mesoderm. The blood- 

 islands are less conspicuous in front, gradually fading away, as they do 

 also on the side next the primitive streak. They are most strongly 

 marked towards the external margin of the mesoderm and here they, 

 as well ai the whole sheet of mesoderm, are being added to by 

 delamination from the endoderm of the germ -wall. The blood- 

 islands, the first rudiments of blood-vessels, are simply thicken- 

 ings and condensations of the mesoderm. They at first have the 

 appearance in sections of occupying its whole thickness but later it 

 is seen that each blood-island is roofed over by a layer of unmodified 

 mesoderm — demarcated apparently by a simple process of splittino--off 

 of the superficial layer of cells. 



As development goes on, the area of vascular rudiments spreads 

 inwards into regions of the mesoderm which have been for some 

 time completely separated from the endoderm by a well-marked split. 



