. The cell-lineage and early larval development of Fiona marina, a nudibranch mollusk. Nudibranchia; Mollusks; Gastropoda. blasts in Crepidula and Fiona is the direct result of epibolic gastrulation in'the one case, embolic in the other, which is in turn caused by the quantity and nature of the yolk which the macromeres contain. An intermediate condition is found in Xereis (Wilson, 1898). Text-figure 1 (a) shows a sagittal section through the cleaving egg of Crepi- dula after one enteroblast has been sepa- rated from the mesoblast. The ectoblast has here but half covered the yolk, and the ento
. The cell-lineage and early larval development of Fiona marina, a nudibranch mollusk. Nudibranchia; Mollusks; Gastropoda. blasts in Crepidula and Fiona is the direct result of epibolic gastrulation in'the one case, embolic in the other, which is in turn caused by the quantity and nature of the yolk which the macromeres contain. An intermediate condition is found in Xereis (Wilson, 1898). Text-figure 1 (a) shows a sagittal section through the cleaving egg of Crepi- dula after one enteroblast has been sepa- rated from the mesoblast. The ectoblast has here but half covered the yolk, and the entoblastic element is thrown down- ward and backward in the direction in which it must go if it follows the ecto- derm over the yolk, and finally reaches a position posterior to the blastopore as that structure is closing (Conklin's fig. 61). In Nereis, text-figure 1 (b), the ec- toderm has advanced much farther over the yolk when the enteroblasts arise, and here we see that these elements are also directed downward but at the same time anteriorly. The next and last step in their change of position is illustrated by Fiona, text-figure 1 (c), in which, on ac- count of its invaginate gastrula, the en- teroblasts are not only anteriorly directed, but also at first He higher than the cells from which they arose. In Trochus (Robert, 1903) the meso- blast arises at about the sixty-four-cell stage by a Isotropic division which sepa- rates the very large cell 4d from 4D. This cell divides dexiotropically and equally when eighty-nine cells are present. When there are one hun- dred and eighteen cells, each of the two derivatives of 4d divides, and of the resulting four cells the anterior pair are the smaller. Later the two larger posterior cells divide. Robert has not found endo- dermal elements to arise from 4d,rbut does not reject the possibility of such a condition. As might be expected from their close relationship, a nearer corre- spondence in the cleavage series is found when we compa
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