The ovum, like that of Eupagurus before segmentation, is now a syncytium. Eventually the nuclei, having increased by division and become very numerous, travel, unless previously situated there, to the surface of the ovum. They then either simultaneously or in succession become, together with protoplasm around them, segmented off from the yolk, and give rise to a peripheral blastoderm enclosing a central yolk mass. In the latter however many of the nuclei usually remain, and it also very often undergoes a secondary segmentation into a number of yolk spheres. The eggs of Insects afford numerous examples of this mode of segmentation, of which the egg of Porthesia[60] may be taken as type. After impregnation it consists of a central mass of yolk which passes without a sharp line of demarcation into a peripheral layer of more transparent (protoplasmic) material. In the earliest stage observed by Bobretzky there were two bodies in the interior of the egg, each consisting of a nucleus enclosed in a thin protoplasmic layer with stellate prolongations. This stage corresponds with the division into two, but though the nucleus divides, the preponderating amount of yolk prevents the egg from segmenting at the same time. By a continuous division of the nuclei there becomes scattered through the interior of the ovum a series of bodies, each formed of nucleus and a thin layer of protoplasm with reticulate processes. After a certain stage some of these bodies pass to the surface, simultaneously (in Porthesia) or in some cases successively. At the surface the protoplasm round each nucleus contracts itself into a rounded cell body, distinctly cut off from the adjacent yolk. [60] Bobretzky, _Zeit. f. wiss. Zool._, Bd. XXXI. 1878. The cells so formed give rise to a superficial blastoderm of a single layer of cells. Many of the nucleated bodies remain in the yolk, and after a certain time, which varies in different forms, the yolk becomes segmented up into a number of rounded or polygonal bodies, in the interior of each of which one of the above nuclei with its protoplasm is present. This process, known as the secondary segmentation of the yolk, is really part of the true segmentation, and the bodies to which it gives rise are true cells. Other examples of this type may be cited. In Aphis[61] Metschnikoff shewed that the first segmentation nucleus divides into two, each of which takes up a position in the clearer peripheral protoplasmic layer of the egg (fig. 52, 1 and 2). Following upon further division the nuclei enveloped in a continuous layer of protoplasm arrange themselves in a regular manner, and form a syncytium, which becomes segmented into definite cells (fig. 52, 3 and 4). The existence of a special clear superficial layer of protoplasm has been questioned by Brandt. [61] Metschnikoff, "Embry. Stud. Insecten," _Zeit. für wiss. Zool._, Bd. XVI. 1866. My own observations on this form accord in the main with those of Metschnikoff.