220 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



is to be cut into sections, the blastoderm should be removed from the glass by a 

 single stroke with a Gillette blade under 95 per cent alcohol. If removed in any of 

 the lower grades of alcohol the specimen will shrink and wrinkle, but after the 

 blastoderm is well hardened in 95 per cent alcohol it will remain perfectly flat 

 through the processes of embedding. If a specimen is thus fixed and dehydrated 

 on the glass, cells which have been studied in the living form can be readily identi- 

 fied in the total mounts or even in sections. I clear the mounts in benzene and oil 

 of wintergreen. The only obvious shrinkage is in passing into the oil, then in an 

 occasional specimen the ectoderm and mesoderm may crack, while the endoderm 

 usually stays intact against the glass. 



In describing the specimens I shall use the number of somites as an indication 

 of the stage of development. It has become evident, however, that there is a very 

 wide variation in the stage of development of the vascular system with a given 

 number of somites, and moreover, that there is a great difference in the develop- 

 ment of the vessels during the interval between the formation of one somite and 

 that of the next. To show the variation of the vessels with reference to the somites, 

 angioblasts may occasionally be seen in a specimen of two somites in the area 

 pellucida, but they are not usually present there until the chick has 4 somites, and 

 never in great numbers until the stage of 5 somites. Again, a blastoderm of 14 

 somites usually has angioblasts but no vessels in the posterior part of the area 

 pellucida, but I have a specimen of 12 somites in which angioblasts in this area have 

 developed into vessels, the endothelium of which is giving rise to blood-islands. 



METHODS OF NUTRITION OF EARLY EMBRYOS. 



In the study of the blastoderm there is one phenomenon associated with the 

 nutrition of the early embryo which must be clearly recognized, not only on account 

 of its importance physiologically, but because in the living specimen it simulates 

 so closely the formation of blood-vessels. This phenomenon I have called endo- 

 dermal blisters. The early blastoderms receive nourishment in two ways: (1) by 

 wandering cells heavily laden with yolk, which become detached from the thick 

 endoderm of the area opaca and wander throughout the embryo. These cells were 

 described by 0. Van der Stricht in 1892 (p. 217) and have been called wandering 

 endodermal cells by Maximow and Danchakoff; (2) by the absorption of fluid 

 substances from the yolk, which occurs in definitely localized areas. If examina- 

 tion be made of any collection of young chick blastoderms that have been stained 

 and mounted with the endoderm against the cover-slip, it will be noted that large 

 numbers of them show hazy lines, especially in the posterior part of the area pel- 

 lucida (fig. 2, plate 1). 



The appearance of the blisters is far more striking in the living form than in 

 fixed specimens; in the former they simulate vessels to such an extent that almost 

 anyone studying living specimens for the first time would regard them as the 

 beginning of the vascular system. As has been said, angioblasts are not found con- 

 stantly in the area pellucida until the chick has 5 somites; an occasional blastoderm 

 may show the very first angioblasts there at the stage of 2 somites, more often at 



