222 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



Thus, to show them best in total blastoderms the specimens should be fixed as soon 

 as they will stick closely on a cover-slip, which is approximately from one-half to one 

 hour. By the time a specimen has been stretched on a cover-slip for four or five 

 hours the blisters have usually all disappeared. If one wishes to convince oneself 

 that these structures are not artefacts, but represent a true physiological process, 

 it is necessary only to cut out a few early blastoderms and examine them immedi- 

 ately in Locke solution under a binocular microscope, which will show the endo- 

 dermal blisters with great clearness. 



The contents of these blisters is nearly always in complete solution, suggesting 

 that they contain a fluid which has been produced by the digestive activity of the 

 endodermal cells. O. Van der Stricht (1899, page 345) described evidences that 

 the cells of the endoderm show a secretory activity, producing the first tissue fluid 

 for the embryo. On the left side of plate 4, figure 14, the blister contains a 

 single wandering cell from the endoderm, while on the right side the space appears 

 empty. The blisters on plate 4, figure 14, are opposite the intermediate zone of 

 the endoderm which marks the border of the area pellucida. The section shows 

 well the three zones of the endoderm, the thin wall of the mid-line, the interme- 

 diate zone and the area opaca. In this section the cells of the endoderm over the 

 blisters are full of globules of yolk. In many specimens this endoderm has been 

 stretched until it is a very thin, fine line. 



The endodermal blisters, which may be found in any blastoderm of the first 

 two days of incubation and perhaps even later, represent the primitive method by 

 which the embryo is supplied with fluid and nourishment in the early stages, and 

 hence are especially frequent and important in the early stages before the blood- 

 vessels develop. They are areas of absorption of the fluids which bathe the tissues. 

 They represent the absorption by endodermal cells of the fluid of the subgerminal 

 cavity and are of great importance as a demonstration of the method of nutrition 

 of the early stages. They show how the tissue-fluid arises, precedes the blood- 

 plasma, and bathes all of the cells of the growing blastoderm. 



All series of sections of chicks throughout the second day of incubation show 

 also the origin of the wandering endodermal cells. The structure of the endoderm 

 of the area opaca is that of a network of cytoplasm with nuclei in the meshes and 

 enormous droplets of yolk in the vacuoles. From this network many individual 

 cells become packed with droplets of yolk and free themselves. The two types of 

 endodermal cells, those fixed in the network and the wandering type, are shown 

 on plate 5, figure 21, and on plate 6, figure 29. In the latter figure the label en. c. 

 indicates one that has wandered to a position dorsal to the mesoderm. These 

 endodermal cells may attain enormous size and may wander to any point in the 

 substance of the embryo. Later they may be found within the blood-vessels and 

 may circulate with the blood-cells. 



APPEARANCES OF THE EXOCCELOM IN THE LIVING BLASTODERM. 



The second phenomenon with which one must become thoroughly familiar in 

 studying the living blastoderm is the varied appearance of the developing exocce- 

 lom. The subject is taken up here from the standpoint of the appearance of the 



