General principles of zoology . FIG. ioo.—Superficial cleavage of an insects egg (Pieris cratcegi). A, division of the cleav-age nucleus ; B, movement of the nuclei to the periphery to form the blastoderm ; C,formation of the blastoderm. (After Bobretzky.) Distribution of the Modes of Cleavage.—Of the fourmodes of cleavage mentioned the superficial one has aninterest from the point of view of the systematist, since itoccurs exclusively in the Arthropods. The other modesof cleavage are distributed as follows: the discoidal hasbeen observed in the majority of the vertebrates and inthe most highl
General principles of zoology . FIG. ioo.—Superficial cleavage of an insects egg (Pieris cratcegi). A, division of the cleav-age nucleus ; B, movement of the nuclei to the periphery to form the blastoderm ; C,formation of the blastoderm. (After Bobretzky.) Distribution of the Modes of Cleavage.—Of the fourmodes of cleavage mentioned the superficial one has aninterest from the point of view of the systematist, since itoccurs exclusively in the Arthropods. The other modesof cleavage are distributed as follows: the discoidal hasbeen observed in the majority of the vertebrates and inthe most highly organized molluscs, the cuttlefishes, while GENERAL EMBRYOLOGY. I87 the equal and the unequal cleavage can be found in all thegroups of the metazoa. Blastula.—Even during the first stages of segmentationthere is formed a cavity, the cleavage cavity, in the interiorof the egg between the cells; with the progress of devel-opment this cavity becomes continually larger. Around itthe cells lie in the form of a one-layered or
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1896
