ORIGIN OF BLOOD-VESSELS IN BLASTODERM OP CHICK. 221 



the stage of 3 or 4 somites, but in all of these the forerunners of blood-vessels are 

 solid clumps of cells without any lumen and hence not simulating vessels at all, as 

 can be seen on plate 6, figure 27. In such a specimen as the one shown on plate 1, 

 figure 2, where the somites are just beginning, I do not think there is any possibility 

 of the presence of angioblasts in the area pellucida, and these spaces can not be the 

 forerunners of the vascular system. The small dark spots seen in the area pellucida 

 (plate 1, fig. 2) are either globules of free yolk or, in one or two cases, a precipitate 

 of the stain. In the living blastoderms in the early stages these blisters, with their 

 highly refractive contours, make the most striking picture in the entire blastoderm. 

 They appear as isolated spaces of varied size and shape; sometimes there are many 

 small vesicles, or again there are a few large confluent ones resembling multilocular 

 cysts. Their walls are either a fine, sharp, refractive line, in which case the spaces 

 are much distended, or the border appears to be wider, limited by a definite row of 

 cells with an occasional nucleus looking exactly like the nuclei of endothelium, 

 bulging into the lumen. Such blisters are to be seen in section in plate 4, figure 

 14, from a chick with no somites and of nearly the same stage as the one shown on 

 plate 1, figure 2. 



The fluid in these spaces is held in place by irregular threads of endodermal 

 cells which stretch toward the mesoderm; as can be imagined, these threads break 

 readily and hence the spaces may change with great rapidity, even disappearing 

 entirely within a few minutes. In order to find them in sections one must fix a 

 specimen while they are still visible, as was done with that shown on plate 4, figure 

 14. Moreover, the identification in sections of blisters that had been recorded 

 in the specimen at the time of fixation is possible only in a technique such as 

 I have described, in which the specimen is kept on the cover-slip throughout 

 dehydration. If the blastoderm be removed with care from the glass the blisters 

 are not usually broken. 



In the living specimen one can prove definitely that these structures are blebs 

 of endoderm by means of the focusing screw, as their contours can be made to lead 

 over to the thin film of endoderm that covers them. Moreover, if they are not too 

 thick one can focus through them to the mesoderm beneath. An excellent descrip- 

 tion of this phenomenon is given in the article of McWhorter and Whipple (1912, 

 page 125), except that these authors interpret it as the beginning of blood-vessels; 

 that is, they include these structures with undoubted vessels which they followed 

 in later stages. They describe them as under the ectoderm rather than under the 

 endoderm, and indeed similar blisters do occur under the ectoderm. By my method 

 of studying the specimen with the endoderm against the cover-slip the blisters of 

 the endoderm are much plainer, and I think that they are much more frequent. 

 McWhorter and Whipple also state that they are not to be found after the stage 

 of 10 somites. I find them frequently in the older specimens, for example at the 

 stage of 17 or 18 somites; and in specimens under 5 somites, I would say that they 

 are almost constant. 



It will be seen readily that these blisters tend to disappear under the experi- 

 mental conditions of growth on a cover-slip and that they can not form again. 



