THE EXTERNAL CONDITIONS OF DEVELOPMENT 325 



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 

 differentiated into cells having the histological character of the normal 

 entoblast ! One of the most fundamental of embryonic differentia- 



Fig. 142. — Regeneration in coelenterates (. /. D. from LoEB ; C. D. from BiCKFORD). 

 ./. Polyp {Cerianthus) producing new tentacles from the aboral side of a lateral wound. 

 B. Hydroid ( Ttibularia) 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 Ttibularia. 



tions is thus shown to be intimately conditioned by the chemical 

 environment. 



The observations of botanists on the production of roots and other 

 structures as the result of local stimuli are familiar to all. Loeb's 

 interesting experiments on hydroids gave a similar result ('91). It 

 has long been known that Tubularia, like many other hydroids, has 



