. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . nt. The process begins at various periods before the closing of the young colonyand is finally completed by a reclosure of the colony, the surfaces of whichare now reversed, at a somewhat later period. 12—2 180 Volvocinea? Reproduction occurs in Pandorina by zoogonidia which are preciselysimilar to the mature cells of Chlamydomonas (Dangeard, 00). They ariseby the longitudinal division of the contents of the mother-cell, and each onesecretes
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . nt. The process begins at various periods before the closing of the young colonyand is finally completed by a reclosure of the colony, the surfaces of whichare now reversed, at a somewhat later period. 12—2 180 Volvocinea? Reproduction occurs in Pandorina by zoogonidia which are preciselysimilar to the mature cells of Chlamydomonas (Dangeard, 00). They ariseby the longitudinal division of the contents of the mother-cell, and each onesecretes in addition to its own membrane a mucous outer coat whichultimately forms the common investment of the colony. Schroder (98) alsoobserved zoogonidia in Pandorina, and states that they lose their cilia,vacuoles, and pigment-spot before dividing to form a new colony. Gamogenetic reproduction has been observed in all the genera exceptPlatydorina and Stephanoon. In Gonium it is by the fusion of isogameteswhich may arise from any cell of the coenobium. In Pandorina the gametesare formed as in Gonium, but they exhibit considerable variability in Fig. 108. A, C, and D, Volvox aureus Ehrenb. A, monoecious sexual colony, x 210; C, twoantherozoids (after Klein); D, ripe oospore, x 475. B, ripe oospore of Volvox ylobator (L.)Ehrenb., x475. a, androgonidia ; an, antherozoid ; <j, gynogonidia. They fuse in pairs, generally a smaller (male) gamete with a larger andmore sluggish (female) gamete (fig. 92 C}. In Pleodorina, Eudorina, andVolvox the differentiation of the sexual cells is altogether more complete,although it is in Volvox that the greatest differentiation is found. In thisgenus there are asexual ( purely vegetative) colonies and sexual colonies, andthe latter, which sometimes possess parthenogonidia also, may be monoaciousor dioecious. There are in the sexual colonies certain reproductive cellswhich become either androgonidia or gynogonidia, the former giving
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