OF THE GEKMINAL LAYERS TN OJASTROPODA. 17 



onwni'd it <;'r:i(lii;illy hocomes evi(l(Mit tluit :ill tlic generations of the 

 oetonioro-cell, as a whole, make but a sliglit displaceiiient compared 

 witli wliat is generally seen in other s^iecies. They do not strictly 

 ov(M'lay tlie eiitonieres ; bnt lie toward the ant(M'ior, and not 

 toward the postei'ior, end as was the case in Aplysi(( already 

 allnded to. 



Aplysia (/^.,, /;,.,, r,.,, r/^ ,, Figs. 51-'")2, PL IIT) : — Tn 

 A. limncrna this generation is said to l)e propagated from tlie 

 tliird quartet of eetomercs ; l)nt in tliis S})ecies it is descended 

 from the first set of ectomeres. Cleavage begins with the cell r, 

 and its course is in a left-handed spiral. Tn Fig. »jl, represent- 

 ing the apical view of such a stage, the new ectornere-cell (',.1 is 

 seen to have been completely l)ndded off from its mother-cell c^ 

 and to have already assumed the normal form ; while in all of the 

 other quadrants the spindle remains in the amphiaster stage. The 

 same phase in the j^osterior two cells of the first ectoraere-quartet 

 is îdso recognizable in tlu; o]^tical transverse section seen from 

 the jiosterior side (Fig. /)2). The new ectomere-cell or tlie trocho- 

 blast is very much smaller than any of the foregoing ccioineres 

 as in the case oïSiphonaria. However in its general outline and in 

 its reaction toward the staining fluids it bears a great similarity to 

 the third quartet of ectomeres in Siphonaria, so that its recogni- 

 tion is a matter of considerable ease. The resemblance of these 

 two generations of ectomeres in the two species is intelligible 

 when we remember that they are both the first direct descend- 

 ants of the first generation of ectomeres and thus have the same 

 genealogical history. The fifth quartet of ectomeres occupies a 

 position alw^ays ventral to its mother-cell and lies almost exactly 

 on the ectomere-cells of the third generation. It also interposes 

 itself between the ectomere-cells of the second generation. 



