54 



VI. 



History of development. 



(Ontogeny). 



I must presume it very probable that the embryonic development of the Brisinga 

 may exhibit peculiarities of very great interest, and perhaps will open for us a still clearer 

 view of the nature of the Echinoderms in general; and I must so much the more regret that 

 I can give no information as to this important point in the natural history of the Brisinga. 



How the ova, when ejected from the ovaries, progress after fecundation; whether 

 they produce, as seems to be the rule for star-fishes, a bilateral symmetrical freely swim- 

 ming larva in which the first foundation of the future star-fish is found, or whether the 

 development takes place more directly, as is the case with a much more limited number of 

 them, must be decided by future investigation. 



We may however by anticipation, and with some degree of confidence, assume the 

 existence of the first named mode of development. My direct investigations as to the deve- 

 lopment do not begin until after that the animal has acquired the radiary form peculiar 

 to the class. 



I. Progressive development. 



Under this head we have to treat of the changes in the appearance of the whole 

 animal, and in the form and structure of the single parts which they undergo in the normal 

 progressive growth, from the time when the animal has acquired its radiary form, until it 

 attains the fully developed state. These changes are in general such as are common to 

 all individuals without exception, and must be well distinguished from the changes in parti- 

 cular details which usually occur later, partly determined by purely, accidental causes partly 

 in consequence of a more or less abnormal development. 



