226 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



is the very beginning of thedifferentiation of the mesoderm of this area into a dorsal 

 or coelomic part and a ventral or vascular layer (a) . 



While it is true that in an occasional section of a specimen of the stage shown 

 in figure 2, plate 1, the very beginning of this differentiation of the coelomic meso- 

 derm can be found in the posterior area, it is not constant or extensive there until 

 the stage of 4 somites. The fact that the formation of the exocoelom is initiated by 

 the arrangement of the mesoderm into two layers of cells was described and illus- 

 trated by 0. Van der Stricht for the rabbit in 1895. This author stated that the 

 mesodermal cells arrange themselves into two layers, a dorsal layer, the forerunner 

 of the somatopleure, and a ventral layer, the forerunner of the splanchnopleure; and 

 that at the start there are no spaces between these two layers, but gradually there 

 appear isolated clefts which flow together to make the cavity of the ccelom. It can 

 thus be made very clear that the ccelom forms from the arrangement of the meso- 

 derm into two layers, which then split apart without the destruction of any of the 

 cells. It is this lack of destruction of tissue that I wish to emphasize, since it 

 brings out in strong contrast the fact that the coelom forms by a method entirely 

 different from the methods by which blood-vessels on the one hand, and the cerebro- 

 spinal spaces on the other, are formed. 



These three structures, blood-vessels, ccelom and the cerebro-spinal spaces, 

 each have a different embryological history and can not be too strongly contrasted. 

 I shall show that the lumen of a blood-vessel forms by the solution or liquefaction 

 of the central part of a solid mass of protoplasm and thus can not be considered as 

 having any relation to tissue-spaces. According to the work of Weed, the arach- 

 noidal spaces form in a mass of mesenchyme. It is interesting to note that in the 

 stages which we are considering in these early chick blastoderms there is no true 

 mesenchyme, but simply mesoderm in layers and angioblasts in solid clumps or 

 masses. Dr. Weed has proved that the arachnoidal spaces come from tissue spaces 

 in a mesenchyme by gradual increase in the size of the mesh of the mesenchyme 

 brought about by the dilatation of existing spaces and by the breaking of some of 

 the strands of the mesenchyme cells. In this process the residual mesenchyme cells 

 ultimately flatten out to make a lining for the cerebro-spinal channels. The same 

 process is to be seen in the formation of the perioticular spaces in the inner ear 

 (Streeter, 1918). On the other hand, the cells which go to make up the ccelom line 

 up in the form of two layers, which then split apart without any destruction of 

 tissue. Thus, embryologically, these three structures are entirely different, and are 

 not analogous in any sense. Blood-vessels develop from dense, solid masses of 

 cells ; the ccelom develops from cells that invaginate to form a mesodermal cavity or, 

 as in the chick, from mesoderm which forms two layers subsequently splitting apart ; 

 while the arachnoidal channels come from the spaces of a typical mesenchyme. 

 Blood-vessels are not derived from tissue-spaces; while, on the other hand, the coelom 

 and the arachnoidal spaces both come from tissue-spaces, but by different processes. 



In a study of the differentiation of angioblasts in the living chick, one must 

 be able to distinguish them readily from the appearances of the ccelom, as seen in 

 figure 4, plate 1, and the point becomes still sharper in the stage of 5 somites 



