ORIGIN OF BLOOD-VESSELS IN BLASTODERM OP CHICK. 225 



plate 1, has been involved in making these vesicles, and very few of the cells have 

 become angioblasts. About the middle of the area opaca, on either side, are certain 

 denser masses of cells which can be identified as angioblasts, but such a specimen 

 emphasizes the value of not calling the first isolated masses of mesoderm blood- 

 islands or even angioblasts, in spite of the fact that so much of the primitive meso- 

 derm is not always involved in forming the coelom. It will be noticed that these 

 angioblasts lie opposite the septa of the vesicles, as was pointed out by Riickert. 



In figure 4, plate 1, is a specimen with 4 somites showing still more differentia- 

 tion. Here the primitive streak is almost without wrinkles, for the specimen was 

 grown on a cover-slip for only 35 minutes. There are two small endodermal blisters 

 in the margin between the area opaca and the area pellucida at the posterior end 

 of the blastoderm. In the zone of the amnio-cardiac vesicles the ccelom is in the 

 form of the large, closely packed vesicles, like those of the area opaca in figure 3, 

 plate 1 . This type of vesicle for the coelom is constant over the area of the amnio- 

 cardiac vesicles and over the area opaca. 



The middle zone of the area pellucida presents an entirely different appearance, 

 here is to be seen another and an important phase in the development of the exocce- 

 lom, a phase with which it is essential to be thoroughly familiar in relation to the 

 study of the vascular system in the living form. It is a stage in which there are no 

 large, definite vesicles, closely packed together like those just described, but rather 

 where there is a delicate network of mesoderm with wide gaps where the mesoderm 

 fails altogether. Such an area is shown in figure 16, plate 4, a photograph of a 

 section passing through the first somite of a chick of 5 somites. About the middle 

 of this section there are two very plain gaps in the mesoderm. The same point is 

 well shown in figure 27, plate 6, from the same area at a later stage, in which there is 

 a network of angioblasts as well as a network of the exocoelom. 



It will be seen in the section on plate 4, figure 16, that the delicate network of 

 mesoderm shown in figure 4, plate 1, over the middle part of the area pellucida, is 

 in the form of two layers with an excessively narrow slit between them, quite 

 different from the wide vesicles of the amnio-cardiac region shown in figure 15, 

 plate 4. In fact, the two sections shown in figures 15 and 16, plate 4, are to be 

 compared with the appearance of the mesoderm in the anterior and middle part of 

 the area pellucida in figure 4, plate 1. In the anterior half of the area opaca, on the 

 other hand, two different structures are visible in figure 4, plate 1 : the large vesicles 

 of the ccelom, and solid bands of cells which tend to lie opposite the edges of the 

 vesicles and which show particularly well on the right side. These bands or cords 

 of cells are angioblasts and, in the specimen itself, are very clear. The posterior 

 part of the area opaca, on the other hand, has dense masses of mesodermal cells 

 which, in the total preparation, seem entirely undifferentiated; that is to say, one 

 can not make out the difference between the coelomic mesoderm and the angio- 

 blasts. In a section of the same stage, however, as shown in figure 29, plate 6, it 

 can be seen that the dorsal cells of the mesodermal masses are just forming two 

 definite layers (mes.) which are the forerunners of the lining of the exoccelom. This 



