1893. EXPERIMENTAL EMBRYOLOGY. 299 



of the pressure. The formation of micromeres was suppressed ; the 

 sixteen-cell stage was a plate ; and yet typical Plutei were formed 

 when the pressure was removed. In other cases, a two-layered 

 plate of sixteen cells was formed, and yet normal Plutei resulted, 

 a fact which seemed to Driesch definitely to contradict the concep- 

 tion of His that there are definite germinal areas. What should have 

 formed one pole of the embryo formed the two sides, and what 

 should have formed the other pole of the embryo formed both poles. 

 In fact, the segmentation-cells of sea-urchins are on to the sixteen- 

 cell stage markedly homogeneous, for even after a very abnormal 

 development normal larvae may result. 



The Hertwigs have shown that when doubly-fertilised the ova of 

 sea-urchins divide simultaneously into four. In such ova, which 

 Driesch believed to have been doubly-fertilised, the whole rhythm of 

 cell-division was double, so that the sixteen-cell stage was not a true 

 sixteen-cell stage (with four micromeres) but a double eight-cell stage. 

 But none of these forms developed. 



VII. MM. Pouchet and Chabry, experimenting in 1889 with the 

 developing eggs of sea-urchins, found that when some of the lime- 

 salts in the sea-water were replaced by potassium or sodium oxalate, 

 the skeleton of the larva was incomplete or entirely absent. An 

 apology for a Pluteus with food-canal and other organs, but without 

 skeleton, was formed, even when nine-tenths of the lime was got rid 

 of. Further reduction of the lime inhibited all development, even 

 gastrulation. 



VIII. Following the researches of Pouchet and Chabry, the Hert- 

 wigs, and others, Herr Curt Herbst of Zurich has recently made an 

 interesting series of experiments on the ova of sea-urchins. To 

 the sea-water in which these were developing he added solutions 

 of various salts, usually in the proportions of 3-8 grms. to 100 cm. of 

 sea-water. Each experiment was checked by a control experiment 

 in which the conditions were the same, excepting the addition of the 

 salt-solutions. Fertilised ova were always used, to avoid the com- 

 plications of polyspermy. The ova were those of SphcerecJiimis 

 granulans, Echinus micvotubevculatiis , and Strongylocentrotus lividus, and in 

 the three cases the results were practically the same. 



Forty-four experiments were made in which so much of the sea- 

 water, e.g., 140 c.cm. out of 2,000 c.cm., was replaced by a yy per cent, 

 solution of KCl in ordinary water, which contained a considerable 

 quantity of lime. So the observer had to deal with the addition of 

 something new rather than with the removal of much lime. 



The formation of the calcareous needles was delayed ; when they 

 appeared they were abnormal and always incomplete. The internal 

 structure of the Pluteus was distinct, but the characteristic processes 

 were small and rounded off. If there was less than 7 per cent, of the 

 KCl solution the abnormality of the larvae was less pronounced. 



These results are not peculiar to KCl, for experiments with 



