i8o 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



tioned whether the outgrowth should properly be called a head. However, 

 even forms like Figure 63, / and /, behave much more nearly like normal 

 animals than do the completely acephalic forms. 



Acephalic forms: In these the anterior cut surface becomes strongly 

 contracted and a small amount of new tissue fills in the concavity and 

 differentiates into a continuation of the lateral margins which bound the 

 contracted cut (Fig. 63, K, L). There is no trace of cephalic ganglia or 

 of eyes, and locomotion usually continues only a short time when induced 

 by strong stimulation. These forms can be distinguished, even from 

 anophthalmic forms, by their locomotor behavior. 

 A pharynx and mouth develop in acephalic forms 

 from prepharyngeal levels (Fig. 63, K) but, so far 

 as observed, never in those from postpharyngeal 

 levels (Fig. 63, L). 



In Figure 65, ^, the parts of the head which are 

 absent in different degrees of teratophthalmiaare ap- 

 proximately those lying between the corresponding 

 broken lines on the two sides of the median plane. 

 Figure 65, B, indicates in the same way the regions 

 absent in teratomorphic and anophthalmic forms. 

 The mediolateral differential susceptibility deter- 

 mines this differential inhibition, progressing from 

 the median region laterally. Particular attention is 

 called to the fact that the inhibition is not specific 

 for particular organs of the head but is regional, in- 

 volving those parts of cephalic organs which happen 

 to lie in the region concerned. Actually, of course, 

 the inhibition occurs before the organs have differen- 

 tiated; but when differentiation occurs, the relation of particular organs 

 to certain regions is clear. When the median head region is inhibited, the 

 median region of the cephalic nervous system does not develop elsewhere 

 but is absent; with inhibition extending farther laterally the ganglionic 

 defect extends farther laterally, the eyes are approximated, or cyclopia 

 or anophthalmia results, and the lateral cephalic lobes show all degrees 

 of approximation to the median line, finally a single median lobe; in 

 anophthalmic forms there are only the rudiments of head development. 

 In short, this differential inhibition of the planarian head shows no rela- 

 tion to particular organs or their parts but simply eliminates regions pro- 

 gressively from the median plane laterally. Other differential inhibitions 

 to be described later show similar characteristics. 



B 



Fig. 65, A, ZJ.— Cor- 

 responding broken lines 

 on the two sides indicate 

 approximately the re- 

 gions most reduced or 

 absent in teratophthal- 

 mic {A), teratomorphic 

 and anophthalmic {B), 

 forms. 



