[30] The best instance of what appears like a polar cell in Arthropoda is a body recently found by Grobben ("Entwicklungsgeschichte d. Moina rectirostris." Claus' _Arbeiten_, Vol. II., Wien, 1879) near the surface of the protoplasm at the animal pole of the summer and parthenogenetic eggs of _Moina rectirostris_, one of the Cladocera. The body stains deeply with carmine, but differs from normal polar cells in not being separated from the ovum; and its identification as a polar cell must remain doubtful till it has been shewn to originate from the germinal vesicle. The more important of the doubtful cases amongst the Rotifera and Arthropoda are the following. Flemming (83) finds that in the summer and probably parthenogenetic eggs of _Lacinularia socialis_ the germinal vesicle approaches the surface and becomes invisible, and that subsequently a slight indentation in the outline of the egg marks the point of its disappearance. In the hollow of the indentation Flemming believes a polar cell to be situated, though he has not definitely seen one. Hoek[31] believes that he has found a polar body in the ovum of _Balanus balanoides_, but his observations are not perfectly satisfactory. [31] "Zur Entwicklung d. Entomostraken." _Niederlandischer Archiv. f. Zoologie_, Vol. III. p. 62. Bütschli, who has expressly searched for the polar bodies in the ova of Rotifera, was unable to find any trace of them, though he found that as the egg became ripe the germinal vesicle became half its original size. In the parthenogenetic eggs of Aphis he also failed to find a trace of polar bodies, though the germinal vesicle, after the germinal spot had broken up into fragments, approached the surface and disappeared. Whatever may be the eventual result of more extended investigation, it is clear that the formation of polar cells according to the type described above is a very constant occurrence. Its importance is increased by the discovery by Strasburger of the existence of an analogous process amongst plants. Two questions about it obviously present themselves for solution: (1) What are the conditions of its occurrence with reference to impregnation? (2) What meaning has it in the development of the ovum or the embryo?