470 



Prof. Allman on the Hydroida. 



among the Hydroida in which all the zooids in an alternation of 

 generations, whether capable of non-sexual reproduction or not, 

 are at the same time sexual. But still the fundamental principle 

 of the law is adhered to ; the heteromorphic zooids in each period 

 are connected with one another solely by gemmation, while the 

 sum of the forms interposed between every sexual act is always 

 exactly the same. 



The phenomena in the present case may be expressed by the 

 following diagram, in which the sign + is intended to indicate 

 reproduction by gemmules, and (V) reproduction by ova ; while 

 G represents the Geryonidan and M the iEginidan element. 

 The total result of the development of every ovum is included 

 within a boundary -line ; and it will be at once seen that it is in 

 every case exactly similar. 



© 



..&c. 



© 



&c. 



&c. 



&c. 



&c. 



&c. 



It is here taken for granted that the development of the ovum 

 in the Geryonidan element is the same as that in the iEginidan 

 element ; and until reasons can be shown to the contrary, we are 

 justified in making this assumption. 



I have also assumed that Haeckel is right in supposing that 

 the Geryonia is developed directly from the ovum, without the 

 intervention of a polypoid trophosome ; but it will be at once 

 seen that there is no evidence of this, and that, though a true 

 metamorphosis may be proved, the earliest stage of the Medusa 

 may yet be that of a bud upon a fixed though as yet undis- 

 covered trophosome. If this be regarded as the true view, the 

 elements which compose each period in the above diagram must 

 be preceded by another, namely the polypoid element, which 

 will then be the immediate product of the development of the 



