2 14 Edmund B. Ifllson. 



a normal 32-cell stage — z. e., consists of two trochoblasts, one 

 rosette cell (i^'^), and its larger sister cell (i^"'), from which 

 one arises one arm of the cross (Figs. 25-27). It should be 

 noted that the rosette-cell almost always appears somewhat too 

 large, which is owing In part to the fact that it is less crowded 

 than in a whole embryo, but undoubtedly in part also as to a les- 

 sened inequality in the division of i\ Such embryos give rise to 

 actively swimming partial larvae, similar in a general way to the 

 corresponding ones in DentaJ'uim. These embryos do not gas- 

 trulate, but close to form pyriform ectoblastic larvae, which bear 

 at the larger end a group of large ciliated trochoblasts, and at 

 the narrower end an apical organ consisting of a group of cells 

 bearing stiffish motionless sensory hairs (Figs. 28-29). It may 

 be clearly seen that the larger end of the embryo is formed of 

 four primary trochoblasts, each bearing a row of powerful cilia, 

 while just above these at one side are two somewhat smaller sec- 

 ondary trochoblasts. The apical organ at this period appears, 

 in most cases at least, to include only two cells from which the 

 sensory hairs radiate like a fan. This differs from the normal 

 apical organ, in which the sensory hairs form a thick tuft directed 

 straight forwards. The radiating arrangement of these hairs 

 in the partial embryos appears to be due to the fact that the apical 

 cells do not extend so deeply below the surface, and retain a 

 rounded form, so that the sensory hairs spread apart like a fan, 

 while in the normal embryo they are crowded together and as- 

 sume a pyramidal shape, the free surface being considerably re- 

 duced. In the partial larvae, too, the sensory hairs appear rela- 

 tively shorter and more rigid than in the normal organ. 



The composition of these larvae is shown with great clearness 

 by allowing the isolated ^s-micromere to develop in calcium-free 

 water, the action of which is more or less restrained by the ad- 

 dition of a certain amount of normal sea-water. All degrees of 

 dissociation may thus be obtained, and among the resulting cell- 

 groups may be found forms like Fig. 30, in which the cells lie in 

 a loose group, yet approximately retain their normal position. It 

 is evident that each of these larvae represents one quadrant of the 

 products of the first quartet, including four primary trochoblasts, 



