12 ADDRESS TO THE BRITISH ASSOCIATION, 1881. 



constitutes the digestive membrane, and the outer the 

 skin. To this form Haeckel gave the name Gastraea. 

 It is, perhaps, doubtful whether the theory of Lankester 

 and Haeckel can be accepted in precisely the form they 

 propounded it ; but it has had an important influence 

 on the progress of embryology. I cannot quit the 

 science of embryology without alluding to the very 

 admirable work on ' Comparative Embryology ' by our 

 new general secretary, Mr. Balfour, and also the ' Ele- 

 ments of Embryology ' which he had previously 

 published in conjunction with Dr. M. Foster. 



In 1842 Steenstrup published his celebrated work 

 on the ' Alternation of Generations/ in which he 

 showed that many species are represented by two 

 perfectly distinct types or broods, differing in form, 

 structure, and habits ; that in one of them males are 

 entirely wanting, and that the reproduction is effected 

 by fission, or by buds, which, however, are in some 

 cases structurally indistinguishable from eggs. Steen- 

 strup' s illustrations were mainly taken from marine or 

 parasitic species, of very great interest, but not gene- 

 rally familiar, excepting to naturalists. It has since 

 been shown that the common Cynips or Gallfly is also 

 a case in point. It had long been known that in some 

 genera belonging to this group, males are entirely 

 wanting, and it has now been shown by Bassett, and 

 more thoroughly by Adler, that some of these species 

 are double-brooded ; the two broods having been con- 

 sidered as distinct genera. 



Thus an insect known as Neuroterus lenticularis, 

 of which females only occur, produces the familiar 

 oak-spangles so common on the under-sicles of oak- 



