. Libro di testo di embriologia. Embriologia. XVII PEOTOCHOEDATA 583 la vescicola pericardica proviene come un solido sviluppo del corno posteriore della prohoscis-caviti/; e il nervo dorsale di alimentazione è delaminato come una striscia solida di ectoderm, in cui i canali si estendono successivamente dall'estremità anteriore e posteriore del collare luhere rimane in connessione con il ectoderm. Davis' conto dell'embriologia del D. pusillus (1908) concorda in molti punti con Bateson il conto dello sviluppo di D. kowalevskii, che abbiamo appena riassunti. L'embrione portelli fuori e inizia a vita libera quando il c

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. Text-book of embryology. Embryology. XVII PEOTOCHOEDATA 583 The pericardial vesicle originates as a solid outgrowth of the posterior horn of the prohoscis-caviti/; and the dorsal nerve cord is delaminated as a solid strip of ectoderm, into which canals extend subsequently from the anterior and posterior ends of the collar luhere it remains in connection with the ectoderm. Davis' account of the embryology of D. pusillus (1908) agrees in many points with Bateson's account of the development of D. kowalevskii, which we have just summarized. The embryo hatches out and begins free life when the collar region is delimited and one pair of gill-pores has appeared. Owing to the fact that the mud was of a more cohesive kind than that in which D. kowalevskii lives, it was possible, at low tide, to take spadefuls of mud containing the burrows intact, and when these burrows were broken open the fertilized eggs were seen clinging to one side of them. A mixture of corrosive sublimate and acetic acid, osmic acid, and Lo Bianco's chrom- osmic mixture, were used for preserving the embryos. The embryos and larvae, which were removed from the burrows, hved and completed their development in vessels of clean water in the laboratory. When the larvae are first hatched they swim in spirals and rise to the surface, but soon tire and drop to the bottom (Fig. 426). These swimming efforts at first occur at regular intervals, but the intervals become gradually longer and longer, and finally the larvae only glide over the bottom. Soon muscular movements are observable in the proboscis, and with the loss of cilia the burrowing life of the adult is begun. These spasmodic upward movements, however, give opportunities to the tidal current to waft the larvae far from their birthplace. The principal points of interest in the development of D. pusillus, as brought out by Davis, are as follows: (1) The cleavage is not quite regular, for, as in the egg of Amphioxus, the upper four blasto- meres of the

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• bookcentury1900
• bookdecade1910
• bookpubli
• booksubjectembryology