[57] EMBRYOGRAPHY OF OSSEOUS FISHES. 511 



fact, at this period, almost if not quite continuous with the hypoblast 

 below; at any rate, it is not fairly differentiated as a separate layer until 

 after the chorda has been clearly defined. Up to this time the muscular 

 mesoblast is quite solid and composed of closely packed equal-sized cells. 

 After this stage has been reached the chorda cells themselves commence 

 to enlarge, and the whole chorda acquires a gradual augmentation of 

 volume, affecting most conspicuously its diameter. It also loses its de- 

 pressed oval form as seen in section and becomes cylindrical. With the 

 Increase in diameter, the chorda cells also undergo other changes of 

 shape, in the course of which they become columnar, with their longest 

 axes arranged transversely to the axis of the chorda itself. They are 

 finally so disposed that in longitudinal sections the chorda cells appear 

 as if they were arranged into a series of disks placed transversely within 

 the chorda sheath, which has by this time appeared, or at the stage of 

 development shown in Fig. 31. The changes which now follow are very 

 singular indeed ; between or within these discoidal masses of chorda cells 

 cavities appear filled with fluid. These cavities, like the disks or cel- 

 lular septa, are also placed transversely to the axis of the notochord, and 

 are at first lenticular in form, but by degrees they enlarge and displace 

 the chorda cells, as if the^^ were being pushed to the notochordal wall 

 or sheath. The protoplasmic basis of the notochord gradually disap- 

 pears from the axis of the organ, until it is wholly replaced by the fluid 

 cavities, which have increased enormously in volume. The walls of the 

 cavities which make up the axial part of the chorda are exceedingly 

 thin, and in just-hatched embryos of several genera I have as yet failed 

 to discover any trace of nuclei in those portions of their walls which ex- 

 tend into the body of the chorda. The walls of the cavities must have 

 been derived from the protoplasm of the cells of the primitive chorda, 

 and their nuclei have probably been transported to the walls of the 

 chorda sheath, where they seem to be very much flattened and spread 

 out upon the inner surface of the outer walls of the great vesicular 

 cells composing the chorda. Lieberkuhn* compared the great vesicular 

 cells of the chorda filled with fluid to the vegetable cell with its parietal 

 layer of protoplasm contained in a cellulose wall, the whole inclosing a 

 large sap cavity. This comparison would seem to be fully borne out by 

 the foregoing description of what may be witnessed in the development 

 of the notochord of osseous fishes. The probability— in truth, the fact- 

 must be this: The lenticular vacuoles which we find to originate within 

 the chorda at an early stage are not developed interstitially between 

 the disk-like tracts of primitive chorda cells, but in the cell substance 

 itself. As the vacuoles enlarge they become covered by a layer of plasma, 

 which becomes gradually thinner as the vacuole enlarges. The fluid 

 contents which are found in chorda cells have been accumulated by a 

 process of transudation from the surrounding tissues. As the vacuolated 



* Ueber Beivegungserscheinuvgen der Zellen. Schr. d. Gesellsch. z. Beford. d gesammt. 

 Naturwissertsch. z. Marburg. Vol. IX, p. 337, 1870. 



