The porifera and coelentera . larged to form a blastoccel. Such cells do occur in Mollusca(Cyclas,^ Lymnaius, Paludina), and probably have to do with theformation of blood-vessels and blood and other skeleto-trophic tissue,though their history has not been traced (see Figs. 9, 10, and Idhis). ^ See Lankester, Development of Mollusca, Phil. Trans. 1S73. THEORIES OF THE CCELOM It is, I think, now certain that they have nothing to do with theformation of coelom. On the other hand, later researches, e.(j. those of Hatschek onPolygordius (see Fig. 11), have confirmed the important view,which I dedu
The porifera and coelentera . larged to form a blastoccel. Such cells do occur in Mollusca(Cyclas,^ Lymnaius, Paludina), and probably have to do with theformation of blood-vessels and blood and other skeleto-trophic tissue,though their history has not been traced (see Figs. 9, 10, and Idhis). ^ See Lankester, Development of Mollusca, Phil. Trans. 1S73. THEORIES OF THE CCELOM It is, I think, now certain that they have nothing to do with theformation of coelom. On the other hand, later researches, e.(j. those of Hatschek onPolygordius (see Fig. 11), have confirmed the important view,which I deduced from Kowalewskys account of the origin ofthe mesoblast in Lumbricus, namely, that the first rudiment ofthe coelom, instead of detaching itself from the archenteron asa pouch or even a solid mass of cells about to split, may separatefrom the archenteric epithelium as a single pair of cells, whichtake up their position in the blastocoel (space between ectodermand endoderm) in this state of naked simplicity (Fig. 11, A), and. -n Fig. 11.—Transverse Sections of the Larva of the Ch^etopod Polvoordius to showTHE Origin of the Ccelomic Pouches from two Primary Cells detached from theArchenteric Epithelium. A, section of an unsegmented larva, just in front of the anus, showing ec*, ectoderm,end, endoderm, and mes, the two primary mother-cells of the coelom. /J, section of anolder larva near the tail, mrs, the cwloniic rudiments formed by the division and growthof the primitive ca?lomic cells; n, forecast of the nerve cowl. C, section of the same larvanearer the head. The splanchnic (.?•7)) and somatic (so) walls of the cnelom have divergedfrom one another forming; the coeloniic cavity. Ji, forecast of nerve cord. (Aft<r Hatschek,from Korschelt ami Ilcider.) then proceed to multiply so as to form a solid mass of cells rightand left (Fig. 11, B), and finally open out as two well-developedccelomic sacs (Fig, 11, C). This is a fine typical instance of precocious segregation, the
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