THE EXTERXAL COXDITIOXS OF DEVELOPMEXT 



429 



The changes thus caused by shght chemical alterations in the 

 water may be still more profound. Herbst ('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-shaped 



Fig. 194. — Regeneration in crelenterates {A, B. from I.OEB; C, D, from BiCKFORD). 



A. Polyp (Cerianf/ius), -prodncmg new tentacles from the aboral side of a lateral wound. 

 B. Hydroid ( 7>/3«/aW<7), generating a head at each end of a fragment of the stem suspended in 

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



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

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

 undergo the differentiation into entoblast than in the normal de- 

 velopment, the ectoblast sometimes becoming greatly reduced and 

 occasionally disappearing altogether, so that the entire blastula is 



