1*^07.] NATURAL SriEXCES OF PHI I. A 1)1 .Ll'III A. 341 



center is covered by the cuticular substance which forms the earhest 

 sliell; l)eneath this sliell is a veiy thin Layer of ecto(hM-in, in which the 

 nuclei are few and far apart. The niariiin of the shell ^laiid, or mantle 

 eilge, is at first circular in outline; then it Ijeconies uniformly elliptical 

 (figs. 29 and 30); finally it continues to extend in all directions, except 

 where it comes into contact with the organ complex on the ventral 

 side of the embryo; here the growth of tlie mantle edge is arrested, and 

 consequently a notch in the developing shell and mantle appears here, 

 which notch grows deeper as the mantle edge extends farther (figs. 

 31-36). 



The area covered by the shell grows larger continually and the yolk 

 appears to slip through the ring formed by the mantle edge, while at 

 tlie same time the blastoderm and its organ anlagen are retained in 

 front of this ring. In this way the yolk slips out of the anterior portion 

 of the embryo, and the tension on the blastoderm being relieved in this 

 manner, the latter is free to undergo the foldings necessary to form the 

 head vesicle, velum, larval kidney antl heart ; at the same time the mantle 

 edge, continuing to grow forward on the dorsal side, gives rise to the 

 mantle fold and mantle chamber. On the ventral side the forward 

 growth of the mantle edge is stopped l)y the organ complex (figs. 35, 36). 



In the earlier stages of this overgrowth, the mantle edge and organ 

 complex are apparently bilaterally S3anmetrical ; in later stages it can be 

 seen that the organs are more developed on the right side (left in ventral 

 view) than on the left, and consequently the notch in the mantle edge 

 is displaced from the median plane toward the right. In this way the 

 assymetry of this gasteropod makes its appearance. Along the left 

 side of the notch (right in ventral view) a cellular thickening of the 

 blastoderm occurs near the mantle edge (figs. 34, 35, CmM.) which gives 

 rise to the columellar muscle, while the shell formed along this portion 

 of the mantle edge is the columella. 



It is well known that in annelids the ectoderm of the trunk is de- 

 rived from the ectomere 2d (= X). In Fulgur, Crepidula and several 

 other gasteropod s, cells derived from the ectomere 2d give rise to the 

 mantle edge and fold and to the layer of cells covering the yolk unfler 

 the shell. This fact suggests that the elongation of the embryo 

 through the ring of the mantle edge may possibly be comparable to 

 the elongation of the trunk of the annelid. 



4. NERVOUS SYSTEM AND SENSE ORGANS. 



In their earliest stages the organ bases may be recognized by the 

 fact that the nuclei are closer together and the i.irotoplasm stains more 

 23 



