HABITS AND LIFE HISTORY OF THE TOADFISH. II05 



Finally, about the time that the color bands make their appearance, the 

 yolk sac takes a certain very definite appearance. The constriction appears 

 at the upper part of the yolk and divides it into a small upper and a large lower 

 bulb (Ryder, 1886). The lower bulb sits inside the remnants of the egg shell, 

 the upper is partly inclosed in the down-growing body walls. As the fish grows 

 larger and can accommodate within its body more yolk, the constriction travels 

 toward the base or point of fixation of the egg. Presently the yolk sac takes 

 on the hour-glass shape to which Ryder (1887) refers and which he figures so 

 well. Finally all the yolk has been driven into the body of the larval fish and 

 there is nothing left save the placenta-like mass of blastoderm filled with blood 

 vessels. Figure 13, plate cxiii, numbers i to 6, shows the various changes in the 

 yolk sac as the constriction travels toward the base. This photograph is made 

 from "killed" eggs in alcohol. 



The explanation — the only one, so far as I know, ever proposed — is to be 

 found in a little-known oral communication of Ryder to the Philadelphia Acad- 

 emy of Natural Sciences on November 4, 1890. In this he makes known 

 the notable discovery that in the yolk sac of the young Opsanus tau there is a 

 layer of smooth (italics mine) muscle, fibers underlying the epidermis and pre- 

 sumably originating from the splanchnic mesoblast. This muscular layer in 

 turn is composed of layers of spindle-shaped fibers. One layer runs around the 

 yolk bag in equatorial fashion, the fibers of the other run at right angles to the 

 first. Given these facts, the explanation of the changes taking place in the form 

 of the yolk sac is at hand. This is a most interesting point, and, in so far as the 

 writer knows, nothing like it has ever been discovered in any other fish, and 

 great credit is due Ryder for working it out. Ryder surmises that possibly it 

 may have a double function, i. e., not merely that of forcing the yolk into the 

 abdomen of the embryo, but likewise of strengthening the yolk bag so that no 

 injury results from the energetic movements of the fish in its attempts to set 

 itself free. To the present writer this seems to be beyond question the correct 

 interpretation. 



THE FREE-SWIMMING YOUNG TOADFISH. 



The little toads which have just torn themselves away from their anchorage, 

 if looked at from above, appear to difl"er from adults only in size; but if viewed on 

 the ventral surface they show the placenta-like stalk to which reference has been 

 made in the preceding paragraph, and to which not infrequently the adhesive 

 disk is still attached. In a few days the disk (if present) drops off', the placenta- 

 Uke stump is absorbed, leaving only a knob-like projection in the jugular region. 

 Later this disappears and our little fish is no longer a larva but a toadfish, dif- 

 fering from its parents only in size and the stage of development of its repro- 

 ductive organs. Numbers 5, 6, and 7, figure 13, plate cxiii, show the various 



