DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 71 



then fix the specimen. The time required for the nuclear changes is much less 

 than the time taken for the cytoplasmic changes. According to M. R. Lewis, the 

 nuclear phase lasts about 5 minutes, while the cytoplasmic change takes about 

 an hour. The facts that not every nucleus divides at the same moment and that 

 the cytoplasmic changes have not been recognized explain the failure to note the 

 rhythm of cell division. 



Using the criteria for distinguishing angioblasts which have just been indi- 

 cated, I will now describe what has been made out concerning the vascular system 

 at the stage of 6 somites, both in the living specimen and in sections which have 

 been made from a blastoderm in which the cells had been charted in the total 

 specimen before the sections were cut. For this description the axis of the 

 embryo may be divided into four zones: (1) that part of the head which is covered 

 by the head-fold, as seen from the ventral aspect; (2) the head between the head- 

 fold and the first myotome; (3) the zone of the myotomes; (4) the zone caudal 

 to the myotomes. As has been described, there is a dense band of tissue on either 

 side of the axis of the embryo which divides the area pellucida into an inner dense 

 zone and an outer thinner zone. The area opaqua, on the other hand, is denser 

 along its outer margin. Beginning with the area opaqua, in its outer margin 

 there is a large marginal plexus of vessels partly filled with blood-cells which 

 cling in large masses to the dorsal wall of the vessels. The blood-cells can be 

 distinguished from the angioblasts by the fact that in the edges of the masses 

 they tend to separate from the mass and have a definitely round contour. Angio- 

 blasts never have a round contour. In this marginal zone the ccelom is clearly 

 seen, with its dorsal and ventral mesoderm, and the ventral wall of the blood- 

 vessels is very plainly distinguished from the endoderm; but the dorsal wall of 

 the blood-vessels is closely attached to the ventral mesoderm, and in places can 

 not be distinguished from it. 



The inner margin of the area opaqua and the outer margin of the area pellu- 

 cida have two definite plexuses: the dorsal plexus of the coelom and the scantier 

 ventral plexus of solid angioblasts. Over the dense area on either side of the 

 myotomes the ccelom is no longer in the form of a plexus, but has a complete lumen; 

 for there the body-cavity is well formed. The plexus of angioblasts covering this 

 area is continuous with a plexus of angioblasts along the lateral margin of the 

 myotomes. Caudal to the sixth myotome, the plexus extends for a short distance 

 along the undifferentiated mesoderm, curving a little to the side. Very interesting 

 appearances are to be made out near the first myotome. Extending forward from 

 the lateral border of the first myotome, the chain of angioblasts representing the 

 aorta can be seen up to the margin of the head- fold, when it disappears under the 

 fold. Opposite the first myotome, and extending forward from its mesial border, 

 there is also a chain of angioblasts along the hindbrain, and this chain of angio- 

 blasts connects with the aorta above the first and between the first and second 

 myotomes. The chain of cells along the margin of the hindbrain I should not recog- 

 nize as angioblasts in sections; but in the living blastoderm they have exactly the 

 appearance of the angioblasts of the aorta and connect with them by slender 

 filaments. 



