15S Papers from the Marine Biological Laboratory at Tortugas. 



may survive and shed their sex cells on a subsequent day. When the gonads 

 have been emptied the medusae settle to the bottom and die. 



DEVELOPMENT. 



The egg-structure. — The newly-laid eggs vary somewhat in size, but 

 average about 240 fx. in diameter. They are blue or blue-gray in color. They 

 show no polar differentiation save that the germinal vesicle is frequently 

 (perhaps always) eccentric toward the future maturation pole. 



The living eggs are too opaque to show much of their structure, and eggs 

 which have been fixed and sectioned often show certain artifacts, the results 

 of the reagents used. It is therefore difficult to determine with certainty 

 what the normal structure of the egg is. In the living egg, as well as in 

 material fixe<.l in various standard fluids there is a peripheral layer of clear 

 protoplasm in which there are few yolk spherules, and these quite small ; 

 this peripheral layer may therefore be considered a normal feature. In the 

 most perfectly fixed material there is a layer of densely packed yolk spher- 

 ules just inside the peripheral layer; while within this is a central area con- 

 taining fewer yolk spherules scattered through a fluid or semi-fluid matrix 

 which probably represents dissolved yolk ( figs. 37, 39 ) . In material fixed 

 in picro-acetic or picro-sulphuric acid the peripheral layer may be separated 

 by a space from the deeper-lying parts of the egg; on the other hand, in 

 material fixed in Hermann's or Flemming's solutions such a space does not 

 appear, and it is evidently an artifact. Furthermore, in picric fixation the 

 central area is more homogeneous in appearance and the yolk is more com- 

 pact than in material fixed in Hermann's or Flemming's fluids. It is difficult 

 to decide which of these fixations represents more nearly the normal condi- 

 tion of the central area, but this is not a matter of much importance, since 

 in all the fixations used the yolk spherules are less densely crowded together 

 in the central area than in the surrounding layer. 



Hargitt (1906) found a concentric arrangement of the cytoplasm of the 

 eggs of Pennaria, after they had been fixed in picro-sulphuric solutions, but 

 he regards this as an artifact, due to poor fixation. However, so far as the 

 peripheral layer of protoplasm is concerned, it has been seen in the living 

 eggs of many coelenterates ; it is therefore not an artifact, nor is it of rare 

 occurrence. As to the central area of the egg, it is known from a study of the 

 living transparent eggs of several genera of medusae and ctenophores that the 

 substance of this area is frequently of a foamy, or vacuolar, character and 

 that it contains only a scanty reticulum of protoplasm and but few yolk- 

 spherules. I conclude, therefore, that both the peripheral layer and the cen- 

 tral area shown in my sections of the Linerges egg are of normal occurrence. 

 The layer of closely packed yolk spherules, which surrounds the central area, 

 is the only layer which has not been observed either in Linerges or else- 

 where in the living egg, but if both peripheral and central areas are faith- 



