TWINNING AS A MODE OF REPRODUCTION 211 



these metameres specialize so as to become sexually 

 mature and gametic reproduction takes place. In a very 

 real sense, therefore, the process of segmentation may 

 be said to represent an asexual phase in the life- cycle of 

 a metameric animal. 



2. Lateral budding is the characteristic asexual 

 method of reproduction of sessile or sedentary organisms 

 in which one end of the primary axis is permanently or 

 semi-permanently attached to the substratum. We 

 find budding of this kind in Porifera and Coelenterata, 

 the two lowest metazoan phyla in which there is no axis 

 of bilateral symmetry. The characteristic feature of 

 this type of reproduction is that at some level of the 

 primary axis a new growing-point arises that has acquired 

 an independence from the dominance of the apical end. 

 This new apical point grows out essentially at right 

 angles to the main axis, although subsequent flexures 

 may alter the angle between the two axes. The new 

 growing-point becomes the apical end of a new zooid and 

 this proceeds to organize its own basal parts. In the 

 coelenterates the formation of a series of asexual zooids 

 into a colony is essentially like what happens in the case 

 of strobilation or in that of metameric segmentation. 

 In both cases the younger zooids remain sexless, but in 

 later stages at least some of the zooids become sexually 

 mature and produce gametes. 



3. Twinning. — ^T winning is a precocious form of 

 axiate reproduction involving a physiological isolation of 

 bilaterally symmetrical halves of the blastoderm and the 

 consequent physical separation of a single individual into 

 two equivalent parts. Gross fragmentation of a blasto- 

 derm without reference to axiate relations, even if it gives 



