. Journal of morphology. tions of ectomeresprobably take up homologous positions in the body. Themacromeres after the separation of the ectomeres and mesoderm(by the same number of cleavages) become entomeres. of ectomeres is invariable (Wilson, No. 64; Heymons, No. 47) in Annelida andMollusca ; all micromeres after the third generation, with the exception of theposterior micromere of the fourth generation (mesoblast), being entodermic. FromBrobetzkys description of Nassa one would conclude that more than three genera-tions of micromeres are formed ; but until it is shown that all are ectomere
. Journal of morphology. tions of ectomeresprobably take up homologous positions in the body. Themacromeres after the separation of the ectomeres and mesoderm(by the same number of cleavages) become entomeres. of ectomeres is invariable (Wilson, No. 64; Heymons, No. 47) in Annelida andMollusca ; all micromeres after the third generation, with the exception of theposterior micromere of the fourth generation (mesoblast), being entodermic. FromBrobetzkys description of Nassa one would conclude that more than three genera-tions of micromeres are formed ; but until it is shown that all are ectomeres, wecannot accept this as evidence against the views upheld here. Similarly it is notcertain as yet just how many generations of ectomeres are formed in , it has not yet been shown for any annelidan or molluscan ovum ofthe oblique type of cleavage that more or less than three generations of ectomeresis formed. ^ In Nereis a dorsolateral strip of trunk epiblast is derived from A and C,according to Fig. I. —Diagram of the cleavage in Clepsine, Nereis, Amphitrite, and Umo. The part insquare brackets is peculiar to Unio. In Clepsine the third generation of ectomeres has not beenfound and the mesoderm is formed frofn the posterior ma. 3. —ec. i., ec. 2., ec. 3., first, second, andthird generations of ectomeres. ma. 1. to ma. 4., Macromeres of different stages, en., Larval mesoblast. som. i., First somatoblast. som. 2., Second somatoblast.
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Keywords: ., bookcentury1800, bookdecade1, booksubjectphysiology, bookyear1887
