Papers . ieth hour and pushes backuntil it comes into contact with the future esophagus, the iimer endof the archenteron having been directed toward the veritral side. Hyhridizfition of Echinoids. 9 In some sections of embryos of an age of 73 hours the mouth openingdoes not seem to be established; in others the opening is is therefore safe to say that the mouth opens about the beginningof the fourth day. The first appearance of skeletal spicules is aboutthis time, these spicules arising in groups of mesenchyme cells atabout the level of the outer ends of the posterior ciliated bands
Papers . ieth hour and pushes backuntil it comes into contact with the future esophagus, the iimer endof the archenteron having been directed toward the veritral side. Hyhridizfition of Echinoids. 9 In some sections of embryos of an age of 73 hours the mouth openingdoes not seem to be established; in others the opening is is therefore safe to say that the mouth opens about the beginningof the fourth day. The first appearance of skeletal spicules is aboutthis time, these spicules arising in groups of mesenchyme cells atabout the level of the outer ends of the posterior ciliated bands. Asthese rods increase in length, the sides of the embryo grow out to formwhat are probably the post-oral arms (figs. 4, b and c), the fenestratedrods in their growth keeping pace with the growth of the arms. The cultures were kept under observation until the end of our staj^at Montego Bay (two weeks). During this time no essential changein the form of the larva took place. The skeletal rods increased in. Fig. 3a, Optical section living Cidaris gastrula; somewhat flattened bypressure of coverslip; 41 hours; enteroccele forming; mesen-chyme cells abundant in blastocoele. X , Section of blastula of Lytechinus; 6 hours; from fixed material. X , Section of blastula of LytechintLs with mesenchyme cells proliferating; 7 hours; from fixed material. X , Optical section hving gastrula of Lytechinus; 20 hours. X 285. length and the inner ends of these rods assumed a more definiteassociation as a framework at the sides of and ventral to the stomach(fig. 56). In the most advanced specimens that have been studiedthe fenestrated rods show as many as 14 openings, and there has beena pronounced increase in the length of the antero-lateral rods. Thebody-rods extend posteriorly on each side of the stomach, while theventral transverse rods have grown toward the median ventral line,although there has as yet been no contact or union between the endsof the rods of opposite sides.
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