. Text-book of embryology. Embryology. XVI ECHINODEEMATA 531 plate {aj), 395). _ At the opposite pole the invagination which is to form the archenteron appears. This is of very small extent in com- parison with the length of the blastula; it does not, at the period of its greatest extension, equal in length half the total length of the embryo. When the development has reached this point the embryo escapes from the egg-capsule and begins to lead a free-swimming life. In Synapta, therefore, the larval life commences with the gastrula stage, not with the blastula as in the other groups stud
. Text-book of embryology. Embryology. XVI ECHINODEEMATA 531 plate {aj), 395). _ At the opposite pole the invagination which is to form the archenteron appears. This is of very small extent in com- parison with the length of the blastula; it does not, at the period of its greatest extension, equal in length half the total length of the embryo. When the development has reached this point the embryo escapes from the egg-capsule and begins to lead a free-swimming life. In Synapta, therefore, the larval life commences with the gastrula stage, not with the blastula as in the other groups studied. The mesenchyme is formed only after the gastrula stage has been reached. It originates, according to Selenka, in two cells given off from the apex of the archen- teron which multiply by division. This is unlikely, it more probable that numerous cells are given off'.. arch Fig. 394.—The 32-cell stage in the segmentation of the egg of Synapta viewed from the side. (After Selenka ) ]?io. 395.—The free-swimming gastrula of Synapta digitata. (After Selenka.) ap, apical thickening ; arch, archenteron ; Up, blastopore. In, the late origin of the mesenchyme the Synapta larva resembles the Asteroid, and the reason for this may be the same in both cases, namely, the absence of a larval skeleton, to the formation of which is devoted the primary mesenchyme of Ophiuroids and Echinoids, which is given off in the blastula stage. The tip of the archenteron bends at right angles and grows towards the dorsal side of the larva; it fuses there with the ectoderm, and a perforation of the fused layers is effected so that the lumen of the archenteron communicates with the exterior. This perforation is the madreporic pore {mp, Fig. 396). After it is formed the horizontal branch of the archenteron is separated from the vertical one and becomes the coelom, whilst the vertical section of the archenteron is the gut. This latter soon becomes divided by constrictions. Please note that these images
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