. The development of the frog's egg; an introduction to experimental embryology. Embryology. Ch. XIV] ORGANS FROM THE MESODERM 149 outer side of each somite by tlie entire somatic layer. Along the ventral and median boundaries of the somites a layer having a loose epithelial character (mesenchyme) is also to be seen. Thus the central mass whicli is to develop into the myotome lies on the median side of the coelom, and is wholly surrounded by an epithelial layer. Frontal sections show that this layer can also be traced inward for some distance between successive somites over both their anterior
. The development of the frog's egg; an introduction to experimental embryology. Embryology. Ch. XIV] ORGANS FROM THE MESODERM 149 outer side of each somite by tlie entire somatic layer. Along the ventral and median boundaries of the somites a layer having a loose epithelial character (mesenchyme) is also to be seen. Thus the central mass whicli is to develop into the myotome lies on the median side of the coelom, and is wholly surrounded by an epithelial layer. Frontal sections show that this layer can also be traced inward for some distance between successive somites over both their anterior and posterior sur- faces (Fig. 44). "Not merely is mesenchyme produced by the thin peripheral layer of the somites, but in anterior regions considerable por- tions of the kernels of the somites also undergo a metamor- phosis in this direction. Thus, if I be not mistaken, a somite immediately in front of somite 1 has been wholly converted into mesenchymatic tissue. The kernel of the succeeding so- mite (somite 1) has given rise to a considerable quantity of mesenchyme, and the process has been manifested, though to a less degree, even in succeeding ; ^ At the time when fourteen pairs of somites are present 2 the cells of the more anterior somites have begun to differentiate into muscle-fibres. The cells of each somite elon- gate in the antero-pos- terior direction a n d become cylindrical in shape, and each extends the whole length of its somite (Fig. 44, B). Each cylindrical cell has at first but a single nu- cleus. Around the wall of the cell a layer of fine fibrilke appears. The original nucleus divides and re-divides into man}' nuclei, which lie scattered throughout the cell. 1 Field ('91). - Four days after fertilization of the egg, when three pairs of gills have Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not pe
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectembryology, bookyear1
