_Epiblast_ without shading. _Mesoblast_ black with clear outlines to the cells. _Lower layer cells_ and _hypoblast_ with simple shading. _ep._ epiblast; _m._ mesoblast; _al._ alimentary cavity; _sg._ segmentation cavity; _nc._ neural canal; _ch._ notochord; _x._ point where epiblast and hypoblast become continuous at the posterior end of the embryo; _n._ nuclei of yolk. A. Section of young blastoderm, with segmentation cavity enclosed in the lower layer cells. B. Older blastoderm with embryo in which hypoblast and mesoblast are distinctly formed, and in which the alimentary slit has appeared. The segmentation cavity is still represented as being present, though by this stage it has in reality disappeared. C. Older blastoderm with embryo in which the neural canal has become formed, and is continuous posteriorly with the alimentary canal. The notochord, though shaded like mesoblast, belongs properly to the hypoblast.] A general idea of the structure of the blastoderm at this stage may be gathered from the diagram representing a longitudinal section through the embryo (fig. 19 B). In this figure the epiblast is represented in white and is seen to be continuous at the lip of the blastopore (_x_) with the shaded hypoblast. Between the epiblast and hypoblast is seen one of the lateral plates of mesoblast, represented by black cells with clear outlines. The non-embryonic lower layer cells of the blastoderm are represented in the same manner as the mesoblast of the body. The alimentary cavity is shewn at _al_, and below it is seen the yolk with nuclei (_n_). The segmentation cavity is represented as still persisting, though by this stage it would have disappeared. [FIG. 20. THREE SECTIONS THROUGH A PRISTIURUS EMBRYO SOMEWHAT YOUNGER THAN FIG. 28 C. A. Section through the cephalic plate. B. Section through the posterior part of the cephalic plate. C. Section through the trunk. _ch._ notochord; _mg._ medullary groove; _al._ alimentary tract; _lp._ lateral plate of mesoblast; _pp._ body cavity.] As to the growth of the blastoderm it may be noted that it has greatly extended itself over the yolk. Its edge in the meantime forms a marked ridge, which is due not so much to a thickening as to an arching of the epiblast. This ridge is continuous with the embryonic rim, which gradually concentrates itself into two prominences, one on each side of the tail of the embryo, mainly formed of masses of undifferentiated lower layer cells. These prominences will be called the caudal swellings. By this stage the three layers of the body, the epiblast, mesoblast, and hypoblast, have become definitely established. The further history of these layers may now be briefly traced.