xvi ECHINODEKMATA 525 



changed into the 16-cell stage, consisting of two tiers of eight ; this 

 16-cell stage then became a normal hlastula and subsequently 

 developed into a normal larva. 



It followed that nuclei, which normally would have been situated 

 at one pole, would now be found at the sides of the larva, and hence he 

 drew the conclusion that the nuclei are indifferent structures and maybe 

 exchanged for one another without altering the course of development. 



The same result can be obtained by subjecting the developing egg 

 to a moderate degree of heat, viz. a temperature of about 25 ; the 

 four lower cells of the normal 8-cell stage will arrange themselves 

 in one plane with the cells of the upper tier. 



When the eggs were freed from their membranes by shaking and 

 then exposed to water freed from calcium, they segmented normally, 

 but the segments fell apart. This peculiar effect of sea- water devoid 

 of calcium has been alluded to in previous chapters. It was first 

 demonstrated by Herbst (1900). When this happened in the 16- 

 cell stage, when micromeres, mesomeres, and macromeres had been 

 differentiated, it was comparatively easy to recognize the fragments, 

 which usually consisted of from two or four cells, by the size of their 

 component cells (1900). It was found that such fragments, when 

 derived from the microrneric pole, generally died, but that when they 

 developed they grew into perfect larvae ; whereas those from the 

 anti-micromeric or animal pole, on the other hand, generally lived 

 and developed into clear, free-swimming blastulae, but could develop 

 no further. 



From this circumstance Driesch drew the conclusion that the 

 substances which are necessary for the formation of the mesenchyme 

 and gut are, in the 16-cell stage, chiefly centred at the lower pole 

 of the egg ; hence when cells from the upper pole are taken they 

 can never form these organs. 



These conclusions were confirmed by cutting the blastulae of 

 Sphaerechinus in pieces by a pair of fine scissors, as was done with 

 the gastrulae of Asterias. In early stages the fragments healed up 

 and formed perfect larvae, but, once the primary mesenchyme was 

 formed, then fragments from the vegetative half could form gut and 

 mesenchyme but not the long apical tuft of cilia, while fragments 

 from the animal half formed the apical tuft of cilia but never could 

 form gut and mesenchyme. 



Sometimes, by the action of the calcium-free water, the two first 

 blastomeres were incompletely separated ; then there resulted a 

 flattened blastula which was oval, not circular in section. The long 

 axis of the ellipse was obviously the line joining the two nearly 

 separated blastomeres. When the first signs of bilateral symmetry, 

 which in this case are the calcareous stars, appeared, they were situated 

 at opposite sides of the short axis of the ellipse. It is thus proved that 

 the first cleavage furrow is transverse to the median plane of symmetry, 

 and that in normal development the first two blastomeres produced 

 from the egg are anterior and posterior, not right and left. 



