138 CHARLES R. STOCKARD 



plexus has here joined the vein and corpuscles are freely passing 

 into the vessels. The sprout from the vein wall is still seen oppo- 

 site the entrance of the middle vessel. The small plexus arose in 

 loco entirely independently and subsequently became connected 

 with the larger vessel which also arose as we have seen from a 

 group of mesenchymal cells. 



Figure 26 shows another lateral view of the head region of an 

 embryo of 67 hours in which the large vein is filled with circulat- 

 ing corpuscles and the beginning of the same plexus followed in 

 figures 23, 24 and 25 is seen lateral of the vein. At this stage 

 the plexus is entirely disconnected and separated from the vein. 



In the formation of the large yolk-sac vein, as well as all other 

 vessels arising upon the yolk, there is nothing to be seen of the 

 forerunning capillary plexus so strongly emphasized by Thoma 

 in the yolk-sac of the chick. There is no selection and dilation 

 of certain channels in the capillary plexus of the teleost's yolk- 

 sac to form the veins. Here the veins seem to arise in rather 

 definite localities and soon expand into their full form after the 

 circulation has become established. 



This method of the formation of vessels was beautifully brought 

 out by Wenckebach ('86) in the early study already referred to so 

 often. He concludes: "Aus diesen Beobachtungen geht hervor, 

 dass Mesoblastzellen durch selbstandige amoeboide Bewegungen 

 die Wande der Blutgefasse des Dotters bilden. " Raffaele ('92) 

 later confirmed this observation and further was strongly of 

 the opinion that in selachians and other vertebrates a similar 

 process of vessel building from wandering cells also takes place. 



From my present studies on the normal and abnormal Fundulus 

 embryos, I can see no way to doubt that the endothelial wall of 

 the primary yolk vessels in the bony-fish is formed by arrange- 

 ments of wandering mesenchymal cells. 



lower part of the camera sketch is shown an independent capillary plexus not yet 

 connected with the vein. H t, heart. 



Fig. 24 The same vessels 2 hours and 40 minutes later. The main vessel has 

 increased in caliber and two branches of the capillary net have joined the vessel. 



Fig. 25 The same vessel three hours later, a sprout is given off opposite the 

 union with the middle capillary and corpuscles now enter all three capillaries. 

 The arrows indicate the direction of blood flow. 



