The cell in development and inheritance . n the whole directly proportional to the ratiobetween protoplasm and deutoplasm. Partial or discoidal cleavageoccurs when the mass of deutoplasm is so great as entirely to preventcleavage in lower hemisphere. This has been beautifully con- ^ See Wilson, 9S, 99, 2. 3/2 CELL-DIVISION AND DEVELOPMENT firmed by O. Hertwig (98), who, by placing frogs eggs in a centrifu-gal machine, has caused them to undergo a meroblastic cleavagethrough the artificial accumulation of yolk at the lower pole, due tothe centrifugal force. While doubtless containing an el
The cell in development and inheritance . n the whole directly proportional to the ratiobetween protoplasm and deutoplasm. Partial or discoidal cleavageoccurs when the mass of deutoplasm is so great as entirely to preventcleavage in lower hemisphere. This has been beautifully con- ^ See Wilson, 9S, 99, 2. 3/2 CELL-DIVISION AND DEVELOPMENT firmed by O. Hertwig (98), who, by placing frogs eggs in a centrifu-gal machine, has caused them to undergo a meroblastic cleavagethrough the artificial accumulation of yolk at the lower pole, due tothe centrifugal force. While doubtless containing an element of truth, this explanation is,however, no more adequate than Balfours rule regarding the relationbetween deutoplasm and rhythm (p. 366); for innumerable cases areknown in which no correlation can be made out between the distribu-tion of inert substance and the inequality of division. This is thecase, for example, with the teloblasts mentioned above, which containno deutoplasm, yet regularly divide unequally. It seems to be inap- A^. ></^^^; ; / B Fig. 173. — Partial or meroblastic cleavage in the squid Loligo. [Watase.] plicable to the inequalities of the first two divisions in annelids andgasteropods. It is conspicuously inadequate in the history of indi-vidual blastomeres, where the history of division has been accuratelydetermined. In Nereis, for example, a large cell known as the firstsomatoblast, formed at the fourth cleavage (X, Fig. 171, E), under-goes an invariable order of division, three unequal divisions being fol-lowed by an equal one, then by three other unequal divisions, andagain by an equal. This cell contains little or no deutoplasm andundergoes no perceptible changes of substance. The collapse of the rule is most complete in case of the rudi-mentary cells referred to above. In some of the annelids, inAricia, where they were first observed,^ these cells are derived fromthe very large primary mesoblast-cell, which first divides into equalhalve
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectcells, bookyear1902
