(b) partial
(3) centrolecithal ova
(a) regular (with segments united in central yolk mass)
(b) unequal (with segments united in central yolk mass)
(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.