THE EXTERNAL CONDITIONS OE DEVELOPMENT 429 



The changes thus caused by shght chemical alterations in the 

 water may be still more profound. lierbst (92) observed, for 

 example, that when the water contains a very small percentage of 

 lithium chloride, the blastula of sea-urchins fails to invaginate to 

 form a typical gastrula, but evaginatcs to form an hour-glass-shai)ed 



Fig. 194. — Regeneration in ccelenterates {A, /?. from f.OF.R ; C, D, from BlCKFORP). 



A. Polyp (Or/a»/'//«j), producing new tentacles from the aboral side of a lateral wound. 

 B. Hydroid ( 7>//7«/a^-/'c?), generating a head at each end of a fragment of the stem susr • ' 1 in 

 water. C. D. Similar generation of heads at both ends, of short pieces of the stem, in E 



larva, one half of which represents the archenteron. the other halt 

 the ectoblast. Moreover, a much larger number of the blastula-cells 

 undergo the differentiation into entoblast than in the nc^-mal de- 

 velopment, the ectoblast sometimes becoming greatl\- reduced and 

 occasionally disappearing altogether, so that the entire blastula is 



