A Guide to the Study of Fishes, Volume 1 (of 2)

by David Starr Jordan

Fig. 97.—Development of Sea-bass, Centropristes striatus (Linnæus). a, egg prior to germination; b, germ-disk after first cleavage; c, germ-disk after third cleavage; d, embryo just before hatching. (After H. V. Wilson.)

In the early segmentation of the germ the first cleavage plane is established, and the nuclear divisions have taken place for the second; in the latter the third cleavage has been completed. As in other fishes these cleavages are vertical, the third parallel to the first. A segmentation cavity occurs as a central space between the blastomeres, as it does in the sturgeon and garpike.

In stages of late segmentation the segmentation cavity is greatly flattened, but extends to the marginal cells of the germ-disk; its roof consists of two tiers of blastomeres, its floor of a thin film of the unsegmented substance of the germ; the marginal blastomeres are continuous with both roof and floor of the cavity, and are produced into a thin film which passes downward, around the sides of the yolk. Later the segmentation cavity is still further flattened; its roof is now a dome-shaped mass of blastomeres; the marginal cells have multiplied, and their nuclei are seen in the layer of the germ, below the plane of the segmentation cavity. These are seen in the surface view of the marginal cells of this stage; they are separated by cell boundaries only at the sides; below they are continuous in the superficial down-reaching layer of the germ. The marginal cells shortly lose all traces of having been separate; their nuclei, by continued division, spread into the layer of germ flooring the segmentation cavity, and into the delicate film of germ which now surrounds the entire yolk. Thus is formed the periblast of the Teleost development, which from this point onward is to separate the embryo from the yolk; it is clearly the specialized inner part of the germ, which, becoming fluid-like, loses its cell-walls, although retaining and multiplying its nuclei. Later the periblast comes into intimate relations with the growing embryo; it lies directly against it, and appears to receive cell increments from it at various regions; on the other hand, the nuclei of the periblast, from their intimate relations with the yolk, are supposed to subserve some function in its assimilation.

Aside from the question of periblast, the growth of the blastoderm appears not unlike that of the sturgeon. From the blastula stage to that of the early gastrula, the changes have been but slight; the blastoderm has greatly flattened out as its margins grow downward, leaving the segmentation cavity apparent. The rim of the blastoderm has become thickened as the 'germ-ring'; and immediately in front of the dorsal lip of the blastopore its thickening marks the appearance of the embryo. The germ-ring continues to grow downward, and shows more prominently the outline of the embryo; this now terminates at the head region; while on either side of this point spreads out tail-ward on either side the indefinite layer of outgrowing
mesoderm. In the next stage the closure of the blastopore
is rapidly becoming completed; in front of it stretches the widened and elongated form of the embryo. The yolk-plug is next replaced by periblast, the dorsal lip by the tail-mass, or more accurately the dorsal section of the germ-rim; the cœlenteron under the dorsal lip has here disappeared, on account of the close approximation of the embryo to the periblast; its last remnant, the Kupffer's vesicle, is shortly to disappear. The germ-layers become confluent, but, unlike the sturgeon, the flattening of the dorsal germ-ring does not permit the formation of a neurenteric canal.

Fig. 98. Sea-bass, Centropristes striatus, natural size. (From life, by R. W. Shufeldt.)—Page 137.

The process of the development of the germ-layers in Teleosts appears as an abbreviated one, although in many of its details it is but imperfectly known. In the development of the medullary groove, as an example, the following peculiarities exist: the medullary region is but an insunken mass of cells without a trace of the groove-like surface indentation. It is only later, when becoming separate from the ectoderm, that it acquires its rounded character; its cellular elements then group themselves symmetrically with reference to a sagittal plane, where later, by their dissociation, the canal of the spinal cord is formed. The growth of the entoderm is another instance of specialized development. In an early stage the entoderm exists in the axial region, its thickness tapering away abruptly on either side; its lower surface is closely apposed to the periblast; its dorsal thickening will shortly become separate as the notochord. In a following stage of development the entoderm is seen to arch upward in the median line as a preliminary stage in the formation of the cavity of the gut. Later, by the approximation of the entoderm-cells in the median ventral line, the condition is reached where the completed gut-cavity exists.

The formation of the mesoderm in Teleosts is not definitely understood. It is usually said to arise as a process of 'delamination,' i.e., detaching itself in a mass from the entoderm. Its origin is, however, looked upon generally as of a specialized and secondary character.

The mode of formation of the gill-slit of the Teleost does not differ from that in other groups; an evagination of the entoderm coming in contact with an invaginated tract of ectoderm fuses, and at this point an opening is later established.