Archive image from page 163 of The development of the frog's. The development of the frog's egg; an introduction to experimental embryology . developmentoffro00morg Year: 1897 148 DEVELOPMENT OF THE FROG'S EGG [Ch. XIV -M is the coelom, or body-cavity, and is at first continued into the segmental jDlate. Tlie cavity in the segmental plate lies be- tween the outer epithelial layer and the inner solid mass of cells. When the medullary plate of the embryo begins to roll in to form the nerve-tube, each segmental plate begins to break up transversely into a series of blocks or mesodermic somites.
Archive image from page 163 of The development of the frog's. The development of the frog's egg; an introduction to experimental embryology . developmentoffro00morg Year: 1897 148 DEVELOPMENT OF THE FROG'S EGG [Ch. XIV -M is the coelom, or body-cavity, and is at first continued into the segmental jDlate. Tlie cavity in the segmental plate lies be- tween the outer epithelial layer and the inner solid mass of cells. When the medullary plate of the embryo begins to roll in to form the nerve-tube, each segmental plate begins to break up transversely into a series of blocks or mesodermic somites. The process begins first in the region anterior to the middle of the embryo (Fig. 43). The mesodermic somites are at first somcAvhat irregular in out- line. The first well-marked somite lies at about the level of the ganglion of the vagus nerve. In front of this there are traces of another somite wliich is partially broken up into loose mesen- chymatous tissue. Still further for- ward, the series of somites is replaced by loose mesenchyme. In the frog the number of head-somites (or structures â '''rrif'rSLl â¢'-««P°'«l!ng to them) is uncertain. MS. Mesobiastic somites. At first the primitive segments or ml Si?''''''- ''''⢠''''' «o'ites are not separated from the lateral sheets of mesoderm, but almost immediately after the segmental plate has begun to break up transversely into somites, these begin to separate also from the lateral mesoderm. This separation appears first in the intersegmental borders. At this time the medullary folds have met to form a closed tube. Posterior to the fourth segment, the segmental plate is beginning to break up into blocks, but these have, as yet, no sharply marked outer or ventral boundaries. The body-cavity of the lateral mesodermal sheet is at first, as we have seen, sometimes con- tinued into the cavity of the segmental plate, but when the constriction of the plate from the lateral sheets takes place, this communicat
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