Ova with food-yolk may be divided into two great groups according to the eventual arrangement of the food-yolk in the protoplasm. In one of these, the food-yolk when present is concentrated at the vegetative pole of the ovum. In the other group it is concentrated at the centre of the ovum. Ova belonging to the former group are known as telolecithal ova, those to the latter as centrolecithal. In each group more than one type may be distinguished. In the first group these types are (1) unequal segmentation, (2) partial segmentation. The features of these three types have been already so fully explained that I need not repeat them here. In the second group there are three distinct types, (1) equal segmentation, (2) unequal segmentation. These two being externally similar to the similarly named types in the first group. (3) Superficial segmentation. This is unlike anything which is present in the first group, and is characterized by the appearance of a superficial layer of cells round a central yolk mass. These cells may either appear simultaneously or successively, and their nuclei are derived from the segmentation within the ovum of the first segmentation nucleus. The types of ova in relation to the characters of the segmentation may be tabulated in the following way: _Segmentation._ (1) alecithal } regular ova } (2) telolecithal } (_a_) unequal ova } (_b_) partial (3) centrolecithal } (_a_) regular (with segments united in ova } central yolk mass) } (_b_) unequal " " " " } (_c_) superficial. Although the various types of segmentation which have been described present very different aspects, they must nevertheless be looked on as manifestations of the same inherited tendency to division, which differ only according to the conditions under which the tendency displays itself. This tendency is probably to be regarded as the embryological repetition of that phase in the evolution of the Metazoa, which constituted the transition from the protozoon to the metazoon condition. From the facts narrated in this chapter the reader will have gathered that similarity or dissimilarity of segmentation is no safe guide to affinities. In many cases, it is true, a special type of segmentation may characterize a whole group; but in other cases very closely allied animals present the greatest differences with respect to their segmentation; as for instance the different species of the genus Gammarus. The character of the segmentation has great influence on the early phenomena of development, though naturally none on the adult form. EXTERNAL FEATURES OF SEGMENTATION. (105) E. Haeckel. "Die Gastrula u. Eifurchung." _Jenaische Zeitschrift_, Vol. IX. 1877. (106) Fr. Leydig. "Die Dotterfurchung nach ihrem Vorkommen in d. Thierwelt u. n. ihrer Bedeutung." _Oken Isis._ 1848.