3l8 PRINCIPLES OF EMBRYOLOGY 



that, as was the case in the hydroids, any particular level of the body may 

 form either a head or a tail according to whether it is attached to an 

 posterior or to a anterior piece. However, the ability to regenerate a head, 

 as measured either by the frequency with wliich this is successfully accom- 

 plished or by the time taken to do so, is not usually constant along the 

 whole length of the worm. Different species fall into a number of classes 

 in this respect. There are some in which no regeneration of a head occurs 

 from any part; in others, of wliich the well-known Dendrocoelum lacteum 

 is one, the anterior end regenerates easily but the abihty to form a head 

 falls off rapidly and has disappeared at the level of the pharynx. In others, 

 the curve expressing the ability to regenerate a head (the so-called 'head- 

 frequency curve') falls off more gradually and even the most posterior end 

 of the body sometimes carries out the regeneration successfully. Some 

 Planaria normally reproduce by transverse fission, a new animal forming 

 in the posterior end of the body of an old one. In these the head-frequency 

 curve, after falling in the region of the pharynx, rises again towards the 

 posterior end. Finally there are some species, such as Planaria velata, in 

 which the ability to regenerate a head appears to be equal along the whole 

 length of the body. There are also tail-frequency curves in all these species, 

 but less is known about them. 



The ability to regenerate a head is, however, not fully expressed merely 

 by a curve which assigns some defmite ability to each body level. There is, 

 in point of fact, a gradient from the midline of the body towards the 

 margins as well as from anterior to posterior (Fig. 14.4). We have to 

 deal with a two-dimensional field of head-forming ability rather than 

 with a one-dimensional gradient of it. The type of regeneration which 

 occurs when the planarian body is cut in more complicated ways can 

 usually be deduced fairly simply from the pruiciple that lq posterior pieces 

 a head forms at that point of the cut surface wliich has the highest value 

 in the head-producing field. To account for the fact that the same point 

 would form a tail if attached to the anterior piece, the simplest assumption 

 seems to be that the regeneration tends to occur in such a way as to carry 

 on the gradient which is already intrinsically present within the fragment. 

 In this way one can understand such peculiar phenomena as the appear- 

 ance of the tw^o heads at the shoulders of the T-shaped cut shown in 

 Fig. 14.5A-D, The appearance of heads in the situation shown in Figs. 

 14.5E and F is not so fully accounted for, since here the regenerating edge 

 is connected directly both with the anterior and the posterior parts of the 

 body, and further subsidiary hypotheses would be necessary before it was 

 clear what we might expect to obtain. There seems at present to be no 

 adequately tested hypothesis which can deal satisfactorily with all the 



