EXPLANATION OF PLATE 2. ix 



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

 with eight proto-vertebrse. 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 ; pe. pellucid area ; op. opaque area. 



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

 embryo, shewing 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 ; so, mesoblast of the soma- 

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

 heart. 



Fig. n. Section through the same embryo as fig. 10, and passing through the 

 orifice of the omphalo-meseraic vein. of. Omphalo-meseraic vein ; other references 

 as above. 



These two sections shew that the heart is entirely formed from the mesoblast of 

 the splanchnopleure ', and that it is formed by the splitting of that part of the meso- 

 blast which has turned to assume its normal direction after being folded in to form 

 the muscular wall of the alimentary canal. In fig. 1 1 the cavities so formed on each 

 side have not yet united, but in fig. 10 they have united. When the folding be- 

 comes more complete the cavities (of, of) in fig. 1 1 will unite, and in this way the 

 origin of the omphalo-meseraic veins will be carried further backwards. In the sec- 

 tion immediately behind section n the mesoblast had become thickened, but had not 

 split. 



