88 



LUCEKNARI.E AND THEIll ALLIES. 



crories of proof, as to its structure, iusensibly merge into each other, and mutually 

 sustain their position. It will become evident at the outset that the internal 

 surface of the genital sacs was originally the free face of the general cavity of 

 the umbella. In the description (^| 135) of the full grown sacs we have pointed 

 to the fact that the eggs are " sunken in pouches formed by folds of the lining 

 wall;" that is, they do not lie beneath tiiis wall, but against its free surface; the 

 same surface that is continuous with the free face of the" lining Avail, gastro- 

 phra/ma, of the general cavity of the umbella. In all other orders of Acalephae 

 the c'gs, or the spermatic mass, are actually covered by the gastrophragma ; and 

 some even by the gastromyoplax and chondromyoplax, and imbedded by the opso- 

 myoplax. We have already noted the advent of the elements of tlie reproductive 

 organs in the previous section, but special observations were not made until the 

 present phase came to hand, and we possess, therefore, a wider range of develop- 

 ment, in a single individual, than we could have obtained previously. 



176. The iniiiatory process in the formation of a saccule consists in an abrupt 

 rising of a thick, ridge-like fold, in the form of a semicircle {figs. 67, 68). The 

 convex side («') of the semicircle faces toward the periphery of the umbella, and 

 the concave side (« ) opens (at /) toward the proboscis. Thus in the very begin- 

 ning one of the prime features, whicli render this order so singular, is established, 

 as if it were an inevitable necessity to the completion of the type. The ridge is 

 not an actual thickening of the Ouphragma, but a clear duplication of this layer as 

 it grows and rises above the general surface. The elevation increases but a little 

 while, and tlien another phase of procedure is introduced {Jhjs. 69, 70) which com- 

 plicates matters to a slight degree. The concave side (J ) ceases to grow as fast as 

 the convex side (*'), and the latter in consequence leans over towards the former, 

 very mucli in the same manner that the crest of an incoming wave arches over its 

 base just as it breaks on the shore. This goes on rapidly, especially midway 

 between the two ends of the ridge, until a distinct hood is formed. This is the 

 condition in which our figure represents the saccule, as seen from two points of 

 view. The wall of the fold (i', i), it will be observed, is no thicker than its basal 

 continuity {i). At this time the cavity of the new organ is quite deep from its 

 broad aperture (.s') to its innermost parietes (« ), Init it is very shallow. 



177. Soon after this the aperture begins to narrow under the influence of the 

 onward growth and contraction of the edge of the hood, and at the same time the 

 cavity enlarges and becomes more nearly alike in breadth and depth. The first 

 results is the formation of a narrow, rounded entrance {fi<js. 71, 72, s), on a level 

 with the circumjacent area, and the second is accompanied by conduplicatures of 

 the inner wall, at a considerable number of closely approximated points, resulting 

 in the formation of little, shallow, wide-mouthed pouches or concavities (s'-) 

 opening into the general chamber («') of the genital. The latter is, at this period, 

 about as broad in one direction as in the other, and not so deep, but rather pre- 

 serves the proportions of a hemisphere attached by its equatorial face. Between 

 the outer wall of the sac and the follicles there is scarcely any space except at the 

 intervals about the latter, A short distance from the entrance, the wall is con- 

 tracted on all sides, so closely as to fill iq) the passage way {s'^) to the interior com- 



