2IO THE PHYSIOLOGY OF TWINNING 



to the axis of polarity, as it is in the majority of free 

 Infusoria and in the first cleavage of Ascaris, Unio, and 

 similar forms, the process is much more like the transverse 

 fission seen in fiatworms and in metameric groups. 

 It differs from twinning in that the daughter-cells are 

 not equivalent and in a lack of mirror-imaging between 

 them. 



B. MULTICELLULAR AXIATE REPRODUCTION 



I. Transverse fission. — ^This type of reproduction 

 involves the cutting off of a new individual by means 

 of a fission plane at right angles to the primary axis. 

 As a rule a new organism is cut off from the basal region 

 of an individual which is elongated in and growing 

 in a posterior or basal direction. Transverse fission is 

 mainly characteristic of free-living as opposed to sessile 

 organisms, in which a posterior zooid, if cut off, is free 

 to break away and lead an independent life. Examples 

 of transverse fission are seen in the formation of new 

 zooids in planarians, in strobilation in the cestodes, 

 and in metameric segmentation in the embryos and 

 larvae, of annelids, arthropods, and vertebrates. In the 

 more primitive types of transverse fission the newly 

 formed individual breaks away from the old and regener- 

 ates a new anterior end or head, becoming ultimately 

 a complete individual. In the case of strobilation the 

 individuals produced by fission, though they sooner or 

 later become independent, are never able to regenerate 

 the head, and hence remain incomplete. In metameric 

 segmentation the new individuals are incompletely cut 

 off and remain attached, as subordinate zooids, to the 

 original head, and the whole series becomes secondarily 

 integrated into a single organism. Later on some of 



