vi ORIGIN OF THE VESSELS 369 



Blood-islands developing in such regions are therefore clearly deriva- 

 tives of the mesoderm and there is no possibility of the endoderm 

 playing a direct part in their formation as might be the case peri- 

 pherally in the region of the germ-wall. < 



The vascular rudiments become joined up by strands of cells to 

 form a network and this network gradually spreads inwards, its 

 extension being brought about by a progressive differentiation in situ 

 from the mesoderm : there is no actual sprouting inwards of the already 

 formed strands of the network as is suggested sometimes by the 

 study of whole blastoderms and as was once supposed to take place. 



Of the network of cell strands which traverses the rudiment of 

 the vascular area the bulkier portions give rise to masses of blood 

 corpuscles surrounded by an endothelial wall, the more attenuated 

 portions to endothelial tubes without any corpuscles in their interior. 

 In the former case the superficial layer of the cells forming the blood- 

 island becomes raised up from the main mass of cells, fluid accumu- 

 lating beneath it in spaces which are at first isolated but later 

 become continuous. The flattened cells which' are raised up repre- 

 sent the endothelial wall while the main mass of cells left behind 

 represent developing corpuscles. It is to be noted that the endothelial 

 wall separates from the mass of corpuscles first below (i.e. on the 

 side towards the yolk) and laterally, so that after fluid has accumu- 

 lated in the rudimentary vessel the mass of corpuscles still remains 

 attached to its, as yet undifferentiated, roof. The narrower strands 

 between the main blood-islands and also all those in the pellucid 

 area, except sometimes a few near its posterior end, give rise simply 

 to endothelial tubes containing fluid plasma. As the circulation 

 begins the masses of embryonic corpuscles gradually break up, first 

 in the region of the sinus terminalis, 1 the individual corpuscles being 

 whirled away by the current and carried to the heart and thence 

 through the circulation. 



The origin of the vitelline network has also been investigated in 

 Elasmobranchs (especially Torpedo) by numerous workers. It agrees 

 in its main features with what occurs in the Fowl. 



As regards the peripheral vessels in general, of the Vertebrata, 

 we may say that they take their origin as chinks within the mesen- 

 chyme filled with a clear fluid secretion (plasma). These chinks are 

 at their first appearance in some cases clearly intercellular while 

 in others they at first have the appearance of intracellular vacuoles. 

 As has already been pointed out in dealing with connective tissue 

 (p. 292) this difference though at first sight impressive loses most of 

 its apparent importance when regarded critically. In this particular 

 case the protoplasmic masses in which the vacuoles appear are as a 

 rule multinucleate and it is clearly impossible to draw a sharp line 

 of morphological distinction between spaces in such masses with 



1 The topography of the vascular area will be found illustrated later in the special 

 chapter on the development of the Fowl. 



VOL. II 2 B 



