. A textbook of invertebrate morphology [microform]. Invertebrates; Morphology (Animals); Invertébrés; Morphologie (Animaux). 226 INVERTEBRATE MOItPHOLOOY. t I : xe X -. eud of the body aud giving rise by division in a transverse plane to rows of cells, the mesoblast bands. Such a pre- cocious segregation of the mesoderm also occurs in the Oli- gochaeta. In a Lumhricus embryo there may be seen near the posterior extremity of the body the two mesoblasts (Fig. 105, m), lying one on each side of the middle line, with the meso- blast-bauds {nib) extending for- wards from them. A little in front of
. A textbook of invertebrate morphology [microform]. Invertebrates; Morphology (Animals); Invertébrés; Morphologie (Animaux). 226 INVERTEBRATE MOItPHOLOOY. t I : xe X -. eud of the body aud giving rise by division in a transverse plane to rows of cells, the mesoblast bands. Such a pre- cocious segregation of the mesoderm also occurs in the Oli- gochaeta. In a Lumhricus embryo there may be seen near the posterior extremity of the body the two mesoblasts (Fig. 105, m), lying one on each side of the middle line, with the meso- blast-bauds {nib) extending for- wards from them. A little in front of them and on either side ma}' be seen another cell {nh), giving rise to a band extending anteriorly, which later on will become differentiated into the ventral nerve-cord, the cells Avhich give rise to it being neuroblasts; Tr,« Ac\K «... ^ \'? r. w^^^^® ^ ^i^*^6 behind and exter- liQ. 105.—hunFACE View OF Pos- n , ,, TEHioK ExiKEMiry OF Embkyo "''^^'>' *° th&iiQ, on either side, two OP Ltimbriciis (after E. B. Wilson), other cells (?ieaud x) occur, giving rise likewise to germ-bands, whose further fate is undecided, though it seems probable that the inner of the two bauds gives rise to the nephridia, the cells being nephro- bJasts. Thus from a small number of cells the entire nervous system, with the exception of the supraoesophageal ganglion (which arises as a local thickening of the ectoderm, comparable to the apical thickeniiigof theTro- chophore), the nephridia and all the other mesodermal tissues arise, the precocious segregation of these organs being carried to an extent only equalled in the Hirudiiiea. Indications of it, however, are found in the Polycha^ta, not only in the meso- blasts but also in a layer of cells occupying the ventral surface of the embryo, and forming the so-called ventral plate, from which the ventral nerve-cord, the nephridia, and some of the musculature seem to arise. A reduction of the number of cells constituting this ventral
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Keywords: ., bookcentury1800, bookdecade1890, bookpublishernewyo, bookyear1896
