58 DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 



the subject of these three short papers, and which were well 

 carried on in the apartments used by him as a Physiological 

 Laboratory. 



EXPLANATION OF PLATE 2. 



Fig. i is taken from the anterior part of the pellucid area of a thirty hours' chick, 

 with four proto-vertebrse. At n is a nucleus with two nucleoli. 



Figs. 2 and 3 are taken from the posterior end of the pellucid area of a chick 

 with eight proto-vertebras. In fig. 3 the nuclei are seen to have considerably in- 

 creased in number at the points of starting of the protoplasmic processes. At n is 

 seen a nucleus with two nucleoli. 



Fig. 4 is taken from the anterior part of the pellucid area of an embryo of thirty- 

 six hours. It shews the narrow processes characteristic of the anterior part of the 

 pellucid area, and the fewer nuclei. Small spaces, which have the appearance of 

 vacuoles, are shewn at v. 



Fig. 5 is taken from the posterior part of the pellucid area of a thirty-six hours' 

 embryo. It shews the nuclei, with somewhat irregular nucleoli, which have begun 

 to acquire the red colour of blood-corpuscles ; the protoplasmic processes con- 

 taining the nuclei ; the nuclei in the protoplasm surrounding the corpuscles, as 

 shewn at a, a'. 



Fig. 6 shews fully formed blood-vessels, in part filled with blood-corpuscles and 

 in part empty. The walls of the capillaries, formed of cells, spindle-shaped in sec- 

 tion, are shewn, and also the secondary investment of Klein at k, and at b is seen a 

 narrow protoplasmic process filled with blood-corpuscles. 



Fig. 7 is taken from the anterior part of the pellucid area of a thirty-six hours' 

 embryo. It shews a collection of nuclei which are beginning to become blood- 

 corpuscles. 



Figs, i 5 are drawn with an ^ object-glass. Fig. 6 is on a much smaller scale. 

 Fig. 7 is intermediate. 



Fig. 8. A transverse section through the dorsal region of a forty-five hours' em- 

 bryo ; ao, aorta with a few blood-corpuscles, v, Blood-vessels, all of them being 

 formed in the splanchnopleure, and all of them provided with the secondary invest- 

 ment of Klein ; /, e, pellucid area ; o, p, opaque area. 



Fig. 9. Small portion of a section through the opaque area of a thirty-five hours' 

 embryo, showing protoplasmic processes, with nuclei passing from the somatopleure 

 to the splanchnopleure. 



Fig. 10. Section through the heart of a thirty-four hours' embryo, a. Alimen- 

 tary canal ; hb, hind brain ; nc, notochord ; e, epiblast ; s, o, mesoblast of the soma- 

 topleure ; sp, mesoblast of the splanchnopleure ; hy, hypoblast ; hz, cavity of the 

 heart. 



