. Chimæroid fishes and their development. Fishes; Chimaeridae. VOLK AND YOLK-ENTODERM IN EARLY EMBRYO. 97 sections tbe pronephric duct appears, at first only on the left side, as an ectodermal keel, beginning about the plane of the 8th somite. Thence, passing backward, it merges with the somatopleure at about the plane of the 12th somite, after mm. In this section the subnotochordal rod appears for the last time. In 00 the notochord dips into the dorsal wall of the gut; and in pp it forms an evagination of its wall. (^)(i and RR are sections through the neurenteric canal, and ss to uu through
. Chimæroid fishes and their development. Fishes; Chimaeridae. VOLK AND YOLK-ENTODERM IN EARLY EMBRYO. 97 sections tbe pronephric duct appears, at first only on the left side, as an ectodermal keel, beginning about the plane of the 8th somite. Thence, passing backward, it merges with the somatopleure at about the plane of the 12th somite, after mm. In this section the subnotochordal rod appears for the last time. In 00 the notochord dips into the dorsal wall of the gut; and in pp it forms an evagination of its wall. (^)(i and RR are sections through the neurenteric canal, and ss to uu through the tail end. Two further details of this stage are shown in figures 74 and 75. The former of a section close to that of fig. 73 ll, the latter from a section close to fig. 73 G, representing only a detail of the extra-embryonic blastoderm lying under the region of the head. Fig. 74 has been given to illustrate the ingress of yolk material through the ventral wall of the gut, for here is seen the wedge of yolk protruding through the thickened mass of yolk-entoderm cells, but under conditions which bespeak the complicated nature of the process. For the rest, there is here not a mere rupture which admits the yolk into the cavity of the gut, but an attendant. Fig. 74.—Detail of section of early embryo stiown in fig. 73 LL. ?/t Yollc plug pressing into cavity of gut; V', V". y"\ layers of yolk of different consistencies. series of changes of which the "rupture" itself is, with fair probability, the terminal member. Thus the wedge-shaped mass of yolk (j') is composed of fine yolk; it next passes through a transitional zone ( r') into the coarse yolk (>'")• And on either side of the wedge lies a layer of very coarse yolk (j/'" ), which obviously comes into close physiological rapport with the neighboring layers, for this thickens as it approaches the yolk-wedge, and here it is filled with nuclei of extraordinary size. Indeed on one side (left) we
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectfishes, bookyear1906
