258 BULLETIN OF THE 



been raised by a slight constriction, forming a swelling on the external 

 sui'face. At this time we can distinguish two layers, eb., kb., in the undi- 

 vided single layer of the former stage, while between them, as they lie 

 one above the other, there is a slight thin crescent-formed space, which 

 later increases in size, and is filled with a third layer. The elevation, 

 apparently three-layered, with the part of the yolk immediately below it, 

 forms a disk-shaped body with concave surface resting upon the spherical 

 egg. This disk hangs downward as the egg floats in the water. In 

 another egg (PI. III. fig. 4) of about the same age, the shallow constric- 

 tion which marks off the disk from the remainder of the egg is somewhat 

 magnified. Although the general outlines of this embryo are distorted 

 (the constriction being too deep), the stage is an interesting one as 

 showing on one side a slight notch which has appeared in the outer 

 layer, eh. The existence of this notch enables us to determine certain 

 primai-y axes, formerly not distinguishable, on the surface of this larva, 

 which have relations to the axis of the adult Agabna. Before pass- 

 ing to this point, let me say that the outer of the two layers is the 

 epiblast, the inner the hypoblast, and the layer of the intermediate 

 chamber the middle layer {mb.), later constituting the gelatinous mass 

 of the hydrophyllium. The custom of looking at the float as a starting- 

 point for reference of organs, and using the terms proximal and distal 

 in reference to this structure, has been adopted in the writings of some 

 naturalists. This nomenclature can as well be followed . here in the 

 larva as in the adult. The float, although in Agalma it is not the first 

 structure to appear, can be regarded in the young, as in the adult, as 

 situated at a fixed point or pole for reference when studying other organs, 

 since in all genera it is the first permanent structure which appears. 



It will be found in the subsequent history of our larva, that the float 

 develops near by a region of the disk opposite to that in which the notch 

 in the outer of the two layers lies. We can approximately say that in 

 PI. III. fig. 4 it will appear just below the indentation on the left hand, 

 as the figure is drawn. The whole of the disk-shaped elevation which 

 has formed on the egg and destroyed its sphericity lies, therefore, on one 

 side of the future float. That side may be called the germinative side, 

 for on it appear one by one all the remaining organs of the Agalma 

 body. They have, however, at first no regularity in the jjosition in 

 which they form. Using the nomenclature which has been suggested, 

 the notch is on the distal side of the disk, as it is most distal from that 

 pole of the ovum later to be occupied by the float. The hemisphere of 

 the ovum which faces the observer may be called the right side, as referred 



