i893- EXPERIMENTAL EMBRYOLOGY. 297 



which was only pubhshed (through Roux) tovvards the end of last 

 year, is the more interesting since Ctenophora are far removed from 

 Amphibia. In both cases there is unequal segmentation of the ovum, 

 and in both cases the formation of the missing half is long delayed. 



IV. L. Chabry, experimenting with the ova of an Ascidian [Ascidia 

 aspersa), obtained results which are, on the whole, similar to those 

 of Roux. His method is to place the ovum in a glass tube, of almost 

 equal diameter, and to puncture one of the first two segmentation- 

 cells, observing it, meanwhile, under the microscope. The cell dies; 

 the survivor forms a typical half-morula, half-gastrula, a right or a 

 left half-larva. If the two anterior cells of the four-celled stage be 

 destroyed, a posterior half individual results. Quarter and three- 

 quarter forms were also obtained. Moreover, Chabry believes that 

 he has been able to detect the cells whose descendants give rise to 

 eye, notochord, attaching papillae, &c. On to the sixteen-cell stage, 

 at least, each cell has a determined destiny, it represents a definite 

 part of the embryo ; if a cell be destroyed, the defect in the larva is a 

 definite one. It may be noted that Roux does not think Chabry's 

 figures of half-gastrulae really justify the title, it seems to him as if 

 a regenerating process had already begun to operate so as to complete 

 the embryo. There is not, however, any revitalising of the injured 

 half-egg, for the injuries are fatal. 



V. Carl Fiedler (i8gi) experimented with the ova of Echino- 

 derms {especially oi Echiiiits micyotnberciilatus), first trying the puncturing 

 method, afterwards following the Hertwigs' method of shaking the eggs. 

 When one of the first two segmentation-cells was punctured, so that 

 it merely lost some of its substance, it did not die : it recovered and 

 divided as usual, except that the cells to which it gave rise were 

 smaller than those of the other side. The blastula was unsym- 

 metrical, the embryo normal. But when one of the first two seg- 

 mentation-cells was punctyred so that the nucleus was fatally injured, 

 the cell died ; the surviving cell formed a half-morula, a half-blastula, 

 and, perhaps, even a half-gastrula. When two cells of the four-cell 

 stage were injured, a half-development also resulted, and that the 

 same whatever pair of cells remained. Therefore, the first four cells 

 are equivalent ; but at the eight-celled stage this equivalence is lost, 

 for different groups of four turn out differently. 



VI. Hans Driesch (1891) also experimented with the ova of sea- 

 urchins, pursuing the shaking method. When the first two 

 segmentation-cells were shaken apart, one of' them usually died, the 

 survivor divided into a half-morula, as was the case in Fiedler's 

 experiments ; but the half-morula formed a closed blastula of less 

 than the normal size. Then followed a small gastrula stage, and a 

 minute Pluteus larva. Thus, from one of the first two segmentation- 

 cells a normal but minute larva may develop. While Roux got half- 

 embryos from half-an-egg, Driesch got half-sized but otherwise 

 complete embryos. 



