136 CHAKLES R. STOCKARD 



begun to pulsate. Here there is no further doubt that these 

 elongate spindle cells are the elements which will make up the 

 endothelial lining of the vessel wall. 



There is considerable variation in the rate of development of 

 the yolk vessels in embryos of the same number of hours. Some 

 individuals may be in the condition just described, while others 

 of this age may have already begun to establish a circulation of 

 the blood. Figure 23 shows the yolk region lateral of the head in 

 another embryo at 67 hours. In this specimen an incipient cir- 

 culation has begun and the cord of cells illustrated in figure 22 

 has now become a small hollow tube sufficiently open to allow 

 the passage of a single file of corpuscles from the side of the 

 embryo around to the venous end of the heart. The individual 

 cells composing the vessel are distinctly seen and their nature 

 is clearly made out with a higher magnification. They retain 

 the same general appearance presented before entering into the 

 vascular arrangement. 



Near this vessel is shown in figure 23 a partially formed vas- 

 cular plexus which is broken in several places and entirely dis- 

 connected from the large vein through which the blood is flowing. 

 There is of course no circulation of fluid in this partially formed 

 plexus. 



Figure 23 was sketched with a camera lucida at 12 M. and 

 about three hours later at 2.45 P.M., figure 24 was sketched from 

 the same field. The main vessel in accord with Thoma's ('93 

 and 7 96) first law of vessel growth has increased in calibre on 

 account of the increased flow and pressure of the circulation. 

 It now permits the passage of three or more corpuscles abreast 

 and is a strongly developed vessel. The former disconnected 

 vascular plexus has grown towards the large vessel and two of 

 the projections shown in figure 23 have now met the wall of the 

 vein and joined with it. One of the first corpuscles from the 

 circulation to enter the plexus is shown in the figure tightly held 

 in the 'small vessel. Immediately opposite this vessel a sprout 

 is seen growing away from the wall of the large vein. 



Figure 25 illustrates the state of arrangement at 6.00 P.M., 

 three hours older than figure 24. The third process from the 



