42 The Development of a Primitive Animal 



sexes ( a single colony produces reproductive polyps of one sex only ) . The 

 male and female varieties are quite differently formed, as seen in Fig. 20. 



Regeneration of Polyps and Medusae 



The decapitation of a polyp is followed within a few days by the 

 appearance of a new head at the cut end complete with mouth and ten- 

 tacles. If a segment is cut from a polyp stem or from a root-like runner 

 and incubated in water, the isolated piece can give rise to a new polyp. 

 Similarly, a piece taken from a medusa, if large enough, can reorganize 

 itself into a complete functional medusa with all parts normal and intact. 



The regeneration is specific. That is, a piece of tissue taken from a 

 polyp regenerates a polyp. A piece taken from a medusa regenerates 

 a medusa. In a recent study, it has been shown that this specificity ex- 

 tends even to the kind of polyp from which the piece of tissue was 

 taken. Cut stems taken from a Hydractinia colony regenerated true to 

 type. Stems of feeding polyps regenerated only feeding polyps. Those 

 from reproductive polyps regenerated only reproductive polyps. Two con- 

 clusions are permissible from these data: 



1. A complete, functional, adult coelenterate contains many cell 

 types (i.e., cnidoblasts, epidermal cells, gland cells, muscle cells, nerve 

 cells, etc.). These results show that even a small piece of coelenterate 

 tissue either contains all the necessary types or can give rise to them by 

 appropriate transformation of one cell type to another. 



2. The pieces of tissue appear to "know" from what kind of adult 

 they came and to be able to reorganize themselves accordingly. 



Compare these results and conclusions with those gained from the 

 study of Acetabularia regeneration described in Chapter 2. Please note 

 that they are essentially identical, save that in one case we deal with a 

 structure whose parts are divided up into cellular packets, while in the 

 other we deal with one whose parts are not so divided but are nevertheless 

 organized in a specific manner. 



Regeneration by stem segments has other interesting features that 

 are summarized in Fig. 22. First, the rate and extent of new head develop- 

 ment depends on the part of the stem from which the segment is taken. 

 If a polyp stem is cut into serial segments of equal size, the one from the 

 end nearest the previous head (distal end) develops fastest and most 

 completely, while that from the lowest ( proximal ) end is the most laggard. 

 In other words, along the stem there is a gradient of potency for the pro- 

 duction of new heads. In addition, one part of a stem in producing a new 

 head can prevent another part from so doing. For example, a stem segment 



