Memoirs: A Contribution to the Embryology of Pieris rapae

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Abstract
1. Maturation of the ovum of Pieris rapae takes place immediately after oviposition. 2. Cleavage begins within the first hour, and of the resultant nuclei those destined to form blastoderm move outwards to the cortical layer; the remainder are left in the yolk as vitellophags. The movement outwards of the nuclei is partly attributed to a streaming of cytoplasm which drags the nuclei to the periphery. 3. The formation of the embryonic rudiment is described, as also are its changes in form, which consist chiefly of an increase in length and decrease in width. These are accompanied by anterior and posterior flexures of the embryo into the yolk. 4. Gastrulation is effected by a process of overgrowth of the middle plate by two lateral plates. This is preceded by differentiation of the middle plate by loss of cells from its whole length to the yolk, and by a shallow invagination developing along the median line of the ventral plate. 5. Endoderm is formed in anterior and posterior rudiments, which develop as proliferations in the positions of future mouth and anus. 6. Yolk-cleavage and amnion and serosa formation are described. 7. All the processes examined in development take place from before backwards.

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