xviii SUMMARY 653 



the shrimp be cut out, a new eye will be regenerated so long as the 

 optic ganglion is left intact, but that it' the optic ganglion be cut out, 

 an antenna-like organ will lie regenerated instead. The only possible 

 explanation of this fact is to conclude that some influence a hormone, 

 in fact emanates from the optic ganglion, which so influences the 

 regenerating ectoderm as to determine that it shall take on the 

 extremely complicated structure of the compound eye. 



A still more striking case has been described by us (MacBride, 

 1911). In the development of the ordinary sea-urchin normally only 

 one water -vascular rudiment or hydrocoele is formed, and this is 

 situated on the left side of the larva. The ectoderm lying above it 

 becomes iuvaginated to form a deep sac the amniotic sac. From 

 the floor of this sac are developed the adult nerve-ring and adult 

 spines. In the centre of the floor an invaginatiou is formed which 

 gives rise to the adult mouth. From the peritoneal epithelium of 

 the left coelomic sac, which lies beneath the hydrocoele, are developed 

 the dental pockets and the teeth which project into them. 



But occasionally a second hydrocoele is developed which is 

 situated on the right side of the larva. When this takes place the 

 ectoderm on the right side of the body becomes invaginated to form a 

 deep sac, from the floor of which a second nerve ring, a second set of 

 adult spines, and a second mouth are developed, whilst the underlying 

 peritoneal epithelium of the right coelomic sac gives rise to a second 

 set of dental pockets. 



Now, although we have good reason to believe that the common 

 free-swimming ancestor of all Echinoderms possessed two hydrocoeles, 

 yet it is as certain as anything can lie that this ancestor did not 

 possess two mouths surrounded by nerve rings, nor did it have the 

 highly differentiated opines and teeth of the Echinoid. The production 

 of a second hydrocoele is probably an ancestral reminiscence, but the 

 production of these other organs cannot possibly be so explained. 



The only conclusion, therefore, that we can draw is that, in the 

 natural development of >Uchi?t,us, the hydrocoele emits a hormone 

 which causes the ectoderm to form the amniotic invagination, and 

 which causes the left coelomic vesicle to give rise to the dental pockets. 

 It follows further, that particular portions of ectoderm and of coelomic 

 wall are not specialized so that they alone can undergo these modi- 

 fications, but that any portion of the ectoderm and any portion of the 

 coelomic wall can undergo similar modifications, provided that they 

 are acted on by the appropriate hormone, as is shown by the effect of 

 the right hydrocoele on the right coelomic sac and on the ectoderm of 

 the right side. Development then is a kind of progressive specializa- 

 tion, due to the influence of one organ on another by means of 

 hormones. 



Professor Langley has pointed out to us (in, literis] that if an 

 animal changes its structure in response to a changed environment 

 the hormones produced by the altered organs will be changed. Tin- 

 altered hormones will circulate in the blood and bathe the growing and 



