[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.
[31] “Zur Entwicklung d. Entomostraken.” Niederlandischer Archiv. f. Zoologie, Vol. III. p. 62.
[32] The instances quoted by Siebold, Parthenogenesis d. Arthropoden, are not quite satisfactory. In Hensen’s case, p. 234, impregnation would have been possible if we can suppose the spermatozoa to be capable of passing into the body cavity through the open end of the uninjured oviduct; and though Oellacher’s instances are more valuable, yet sufficient care seems hardly to have been taken, especially when it is not certain for what length of time spermatozoa may be able to live in the oviduct. For Oellacher’s precautions, vide Zeit. für Wiss. Zool., Bd. xxii., p. 202. A better instance is that of a sow given by Bischoff, Ann. Sci. Nat., series 3, Vol. II., 1844. The unimpregnated eggs were found divided into segments, but the segments did not contain the usual nucleus, and were perhaps nothing else than the parts of an ovum in a state of disruption.
[33] Darwin, Cross- and Self-Fertilization of Plants, p. 443.
[34] Mr J. A. Osborne has recently shewn (Nature, Sept. 4, 1879), that the eggs of a Beetle (Gastrophysa raphani) may occasionally develop, up to a certain point at any rate, without the male influence.
[35] Dicyema, which is an apparent exception, has not yet been certainly shewn to develop true ova. If its germs are true ova it forms an exception to the above rule.
CHAPTER III.
THE SEGMENTATION OF THE OVUM.
The immediate result of the fusion of the male and female pronucleus is the segmentation or division of the ovum usually into two, four, eight, etc. successive parts. The segmentation may be dealt with from two points of view, viz. (1) the nature of the vital phenomena which take place in the ovum during its occurrence, which may be described as the internal phenomena of segmentation. (2) The external characters of the segmentation.