. Fresh-water biology. Freshwater biology. S82 FRESH-WATER BIOLOGY and the species is carried over the winter in the resting eggs re- sulting from fertilization by the males. In the winter rotifers, the males appear in the spring, and the species is carried over the sum- mer by the resting eggs. By the greater number of rotifers the eggs are laid as soon as they are completely formed, development taking place outside the body of the mother. But some rotifers are viviparous, the egg being retained in the mother's body until it is partly or completely developed. It is remarkable that the vivipar
. Fresh-water biology. Freshwater biology. S82 FRESH-WATER BIOLOGY and the species is carried over the winter in the resting eggs re- sulting from fertilization by the males. In the winter rotifers, the males appear in the spring, and the species is carried over the sum- mer by the resting eggs. By the greater number of rotifers the eggs are laid as soon as they are completely formed, development taking place outside the body of the mother. But some rotifers are viviparous, the egg being retained in the mother's body until it is partly or completely developed. It is remarkable that the viviparous condition is found in several different groups of rotifers that are not closely related, so that it must have been developed independently several times within the Rotifera. Asplanchna and Rotifer are among the best- known viviparous genera. Philodina and Callidina, closely related to Rotifer, as a rule deposit the eggs undeveloped, though certain species in both these genera produce living young. Thus ovi- parity and viviparity, which in some higher animals distinguish grand classes, are among the rotifers both found in the limits of a single genus. It is a rather remarkable fact that the cleavage and early de- velopment of the rotifer egg does not resemble that of the animals to which the rotifers have often been considered the nearest rela- tives. In annehds and lower mollusks the early development shows a remarkable similarity Fig. 869. Developing egg of a rotifer, Asphnchm even in the details of tlie Spiral herrickii de Gueme. o, Single cell stage; b, four ^ ceUs; c, twenty ceUs;i, ninety-four cells; e, optical clcaVage. But in thc rotifcrS section through embryo formed of many cells. ° ,iiv,io (After Jennings.) the clcavagc follows a Completely different type (Fig. 869). The developing rotifer forms a solid mass, which contains no cavity until the organs formed within this mass begin to separate, just before the rotifer takes its final form and becomes active. Wh
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectfreshwa, bookyear1918
