SUMMARY OF CONCLUSIONS. 215 



2. The normal formation of a double embryo from the 

 ovum, in the case of the Polyzoa. 



3. The variable character of the gemmation in the Cystic 

 Entozoa, commonly solitary, as in Cystkercus ; but multiple 

 in some, as in Coenurus and EcMnococcus. 



4. The co-existence among the Echinodermata of cases 

 of ordinary embryogeny, with others, in which the embryo, 

 though still solitary, has a character of distinctness from 

 the primary matrix, or so-called larva, wliich has led to its 

 being regarded as an independent zooid. 



XIII. — To the gamomorphic form of alternation, a similar 

 parallel may be traced in the development of the reproduc- 

 tive organs of the higher species. Such a correspondence 

 is suggested in particular by the follo^ving considerations : — 



1. The periodicity and lateness of development of the 

 organs of reproduction in most species; and their greater or 

 less independence of the rest of the system in some cases, 

 as in the Polyzoa. 



2. The transition, among nearly allied species of certain 

 families, from cases in which the reproductive organs are 

 integral parts of the system, to others in which they occur 

 in detached zooids having the character of distinct and 

 well-ora;anized animals. 



.3. The accidental or non-essential nature of the charac- 

 ters of detachment and organization, which principally 

 distinguish these zooids from common organs of reproduc- 

 tion, as is illustrated in the orders just referred to ; and as 

 is indicated, further, in respect of the latter point, by the 

 contrast of such forms as the rudimentary males of the 

 Cirripedia and Rotifera, which, though truly distinct ani- 

 mals, have their structural development limited almost 

 exclusively to the generative organs. 



4. The correspondence traceable between the emhryosacs 

 of the onules in phanerogamic plants, and the archegonia 

 in the prothalUa of ferns and allied Cryptogamia. 



