THE EVELOPMRNT OP OPHIOTHRIX FBAGILIS. 



573 



a normal feature ot" the development of that species, and ho 

 suggests (as he did not have the earlier stages) that it is an 

 indication that the archenteron is formed, not by invagination, 

 but by the hollowing out of an originally solid endoderm. It 

 is but just to record that when I gave an account of the 

 abnormal development of Ophiothrix fragilis before the 

 American Society of Zoologists in Philadelphia in 1903, Dr. 

 Grave, in commenting on my paper, stated that he had 



Text-iig. 1. 



Ic. 



]a.-lc Three stages in development of eggs artificially fertilised. 

 Id.-if. Three stages in development of eggs naturally fertilised. 

 Mes. Mesenchyme. C(b. Coelomic rudiment. (From the ' Proceed- 

 ings of the Uoyal Society,' B, vol. 79, 1907.) 



obtained further material of Ophiura brevis, and that he 

 had become convinced that the course of events in that 

 species was identical with what I had described for Ophio- 

 thrix fragilis, in that form of the development which I at 

 that period took to be the normal one. 



The contrast between the normal and abnormal methods of 

 development is well shown in text-fig. 1. 



Turning now to the question of the cause of the difference 

 between normal and abnormal development, it will hardly bo 



