. Chimæroid fishes and their development. Fishes; Chimaeridae. EXTRA-EMBRYONIC BLASTODERM. ^. The complexity of the foregoing conditions (fig. 70) appUes us well to the yolk region as to the blastoderm itself. Without enter- ing into undue detail we may note the following: Extreme vacuo- lization of the subgerminal region (the vacuoles at the right in the figure are indicated by dotted lines); they usually occur in or in association with the lighter areas of the germinal yolk. If we regard the vacuoles in the earlier stage as retaining the character of intercellular spaces, they have by this t
. Chimæroid fishes and their development. Fishes; Chimaeridae. EXTRA-EMBRYONIC BLASTODERM. ^. The complexity of the foregoing conditions (fig. 70) appUes us well to the yolk region as to the blastoderm itself. Without enter- ing into undue detail we may note the following: Extreme vacuo- lization of the subgerminal region (the vacuoles at the right in the figure are indicated by dotted lines); they usually occur in or in association with the lighter areas of the germinal yolk. If we regard the vacuoles in the earlier stage as retaining the character of intercellular spaces, they have by this time undergone, in part at least, change of function, serving now as nutriment purveyors to the yolk ento- , blast. In this connection we find that at various points, 10, the coarse yolk is traversed by fine yolk in rifts, whose shape suggests that of the vacuoles oi earlier stages. In this fine yolk, more- over, manv nuclei are present, and, judg- ing from nunierous amitoses, dividing rapidly. In addition to these rifts of line yolk, we note that there occur at many places throughout the coarse yolk small areas of fine yolk, 1 1; these have in nearly every case nuclei in or near them, and we have thus ground for regarding the yolk region of the egg not as a syncytium ///;-, but rather as a mass of yt)lk-filled cells whose boundaries have brt)ken down, but w^hose individuality as cells has not vet been wholly lost. A second section of the extra-embry- onic blastoderm of this stage is shown in fig. 70 A, a detail of a section passing through the blastoderm considerably in front of the preceding section. Here is indicated even better than before the presence of giant cells which have arisen from the yolk, migratetl outward, and are undergoing division in the region immediately l)elow the ectoderm. At one point {a) a yolk cell of gigantic size is shown (unshaded); at other points (A l>, /^) similar yolk cells are undergoing division by amitosis. Figs. 70 A and B. Seclions o( st
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectfishes, bookyear1906
