TYPES OF VERTEBRATE AND INVERTEBRATE EYES 37 



by environmental factors ; and so far as the experimental material is 

 concerned are not due to germinal variations — in other words, the defects 

 have not been inherited and are not due to defects in either of the two 

 germinal cells. Moreover, it seems probable that the occurrence of 

 identical defects in mammals and in the human subject may also be attri- 

 buted to injury or faulty nutrition affecting the embryo in the very early 

 stages of development, although obviously it does not exclude the possi- 

 bility of a defective condition of either of the germinal cells being the cause 

 of a defect in the subsequent growth of the embryo. 



f> 



\J 



A 



C 



D 



Fig. 29. — Various Degrees of Regeneration and Head Differentiation after 

 Amputation of the Head Region in Planaria. (After Child.) 



A — normal ; B — partial fusion of eyes ; C — cyclops eye, lateral sensory pro- 

 jections approximated and directed forwards ; D — cyclops eye, lateral sensory 

 projections fused ; E — anophthalmos, head reduced in size. 



The experiments carried out by Child on Planaria (Fig. 29) and by 

 Stockard on Fundulus (Fig. 30), both indicate that the use of depressant 

 agents produces what is known as " differential inhibition " on growth 

 processes. Thus the most actively growing parts at particular periods 

 of development are the most susceptible to the influence of the drug 

 and suffer most. One of these areas, namely, the interocular and adjoin- 

 ing regions, may be specially mentioned as being a zone where active 

 proliferation and differentiation is taking place, and as being more 

 affected or susceptible to injury than the rest of the head, both in the 

 regenerating head after amputation of the head region in Planaria 

 and in the intact ova of developing Funduli during the gastrular stage of 

 development. 



Incidentally it may be mentioned here, with reference to the discussion 

 which follows on the nature of double-monsters, that two-headed Planaria 



