OF THE GERMINAL LAYERS IN GASTKOrODA. 21 



tioiis taken up l)y tlint quartet of ectomeres which are ])roduced 

 as tlie first descendant of the fourth generation. Subsequently, 

 all the cells come by degrees to shift their positions so as to 

 accommodate themselves to this new generation. Thus the egg 

 has ))een said to ultimately assume a bilateral synnnetry, and as 

 this takes place only very gradually, it is said that it is some- 

 times quite im])Ossible to point out with exactness the transitional 

 stage. In Siphonaria tliis phenomenon happens at, or a little 

 after, the stage in which the seventh generation of ectomere-cells 

 has l)een budded oft'. The alteration of the body-form is in this 

 species also induced by the same generation of ectomeres that 

 causes the change in the other species alluded to above {viz. 

 the first descendant of the fourth generation) ; but what I make 

 out respecting its formation and disposition is totally difterent 

 from what has hitherto been stated. The two poderior quadrants 

 of the fourtli cctomere-quartet (c^ and (?;•), ((djüiitiii(j laterally 

 the mesoine re-cell M begin to present the spindles [Figs. 20 ami 21), 

 tJie left component {d-.') taking the lead. Unlike all others Uicsc 

 spindles lie in a strictly radial and horizontal direction. Con- 

 sequently the daughter-cells, Ic and re when divided, come 

 each to occapy a position more tnedian than th(d of the mother- 

 cell, close to the sides of the mesomerc-cell JI and also they are 

 in exact bilateral symmetry with regard to each, other (Figs. 

 24 and 25). They are quite like the ectomere-cells of the 

 seventh quartet in size as well as in general appearance. The 

 two anterior (piadi'ants a.^ and b.; of this fourth generation, 

 which are so often tlescribed as dividing almost synchronously 

 with, or a little later than, their posterior component cells, never 

 present in this species any trace of a spindle until after the 

 complete differentiation of the germinal layers. On account of 



