20 THE PELAGIC STAGES OF YOUNG FISHES. 



to, and a third parallel with the first, lead to the four-cell and eight-cell 

 stages (Figs. 5, 6, 7). The sixteen-cell stage is reached by grooves running 

 parallel with the second cleavage-plane, beginning as shown in Fig. 10, and 

 ending by producing an oblong disc of cells arranged as in Figs. 8, 9, 11, 12. 

 This form of the disc is often preserved through a considerable number of 

 the succeeding stages, but sooner or later passes into the circular form of 

 Figs. 14 to 18. The direction of the individual cleavage-planes becomes 

 more and more difficult to follow in detail after the sixteen-cell stajre is 

 reached, and beyond the sixty-four-cell stage they can only be traced in a 

 general way. In passing from the sixty-four-cell stage some of the inferior 

 marginal cells become split off from the blastodisc, and by the time the 

 stage of Fig. 18 is reached they cease to have any distinct boundaries, and 

 we see only a wreath of nuclei around the edge of the disc. A few hours 

 after the first appearance of this wreath of nuclei in the layer of protoplasm 

 which we have designated as the periblast, the blastodisc enters upon a series 

 of transformations, which culminate in the formation of the embryo. The 

 most conspicuous part of these changes consists in an expansion of the 

 blastodisc, accompanied in its first stage by an infolding or ingrowth of the 

 marginal cells, which gives rise to the embryonic ring shown in Fig. 19. 

 This ring is composed of two layers, — an upper (ectoderm) and a lower 

 (entoderm and mesoderm); and from it, by a process of axial concentra- 

 tion, which is in principle a concrescent growth, the embryo is formed. The 

 formation of the ring requires only a few minutes ; and before it attains its 

 full width, we notice that there is a more rapid growth at one point than 

 elsewhere (Fig. 21). It is at this point that the axial concentration is taking 

 place to form the embryo. The axial portion has a rapid centripetal growth, 

 but also lengthens backward in proportion as the circumcrescent or epibolic 

 growth of the blastoderm advances. The lengthening of the embryo, and 

 the concomitant expansion of the blastodisc, which carry the embryonic ring 

 over the yolk-sphere and end by bringing the halves of the ring completely 

 together at the hind extremity, are shown in successive stages in Figs. 21 

 to 26. The nature of the processes by which the embryonic ring is grad- 

 ually converted into a bilateral embryo will be considered in detail in the 

 third part of this memoir, and hence the controversies on this point need not 

 occupy us here. In the same place we shall consider the broad embiyonic 

 plate stretching out on either side of the embryo in Fig. 24, and its relations 

 to the embryo. By the time the ring closes (Fig. 20). scarcely a trace of 



