194 INTERNAL FACTORS IV. i 



From the fact that the four animal cells tog-ether may form a 

 Pluteus while each individually fails to pass beyond the gastrula 

 stage, Driesch has argued that this failure depends solely on lack 

 of sufficient material, not on the want of any specific gut-forming 

 substance, and has supported his contention by other evidence. 

 By placing the eggs of Echinus in dilute sea-water the third 

 division was made unequal ; the cells were then isolated in calcium- 

 free water. The large cells thus formed must contain either 

 more of Boveri's pigment-ring (or rather of the substance of the 

 vegetative hemisphere) or else more of the animal hemisphere, 

 probably the former, since a larger percentage gastrulate than is 

 usual with animal blastomeres. According to Boveri, however, 

 all ought to gastrulate if all contain the specific substance for the 

 archenteron. Further, these large cells are able to form mesen- 

 chyme even when the gut is lacking although they possess the 

 middle region of the egg in any case and may lack the micromere 

 area. 



Driesch also points out that a i blastomere and a | vegetative 

 blastomere have both the same amount of Boveri's pigment-ring 

 and mesenchyme area, the latter, however, only half as big a gut 

 and half as many mesenchyme cells as the former, and lastly that 

 i and -5^ animal blastomeres will not only gastrulate but form 

 mesenchyme as well. 



The limitation of the potentialities of ectoderm and endoderm in 

 later stages has been investigated also by Driesch by observing 

 the development of pieces or fragments of the blastulae and 

 gastrulae. These pieces are obtained by cutting or shaking. 

 Loeb has employed dilute sea-water to make the blastula swell, 

 protrude through its membrane and become constricted into two. 



In Echinus, Sphaerechinus, and Asterias, any piece of a blastula 

 when first cut out is crumpled, but soon becomes rounded, and 

 swims about and eventually gastrulates. 



Monsters exhibiting a certain degree of duplicity have been 

 pi'oduced by Driesch by shaking the egg of Asterias in the 

 two-celled stage (this produces apparently a partial separation 

 of the blastomeres) and by placing the blastulae of Echinus in 

 diluted sea- water. In the latter case the gut is single, the 

 skeleton double; in the former the gut is doubled, though 

 the two may subsequently fuse, or even trebled, or may be 



