74 



THE CELL. AND PROTOPLASM 



bition than the polar pattern. Echino- 

 derm larvae with ventrodorsality thus ob- 

 literated become completely radial forms 

 and never show any indication of ventro- 

 dorsal differentiation. With obliteration 

 of all axiate pattern, the organism remains 

 spherical and even after recovery follow- 

 ing return to natural environment shows 

 no axial differentiation, though it may re- 

 main alive for a long time. Embryonic 

 stages of certain hydroids, however, after 

 being made anaxiate in this way can ac- 

 quire a new axiate pattern by resting 

 undisturbed on the bottom of the container. 

 Hydranth and stem develop from the 

 upper, holdfast from the surface in con- 

 tact, probably in consequence of the oxygen 

 gradient. 



Interesting mediolateral differential in- 

 hibitions occur in various animals from 

 planarians to vertebrates. The normal 

 planarian head is triangular with two 

 bilaterally localized ganglionic masses con- 

 nected with each other, two eye spots 

 dorsal to them and a sensory lobe or 

 auricle at each side of the base of the tri- 

 angle. With increasing degrees of inhibit- 

 ing action, inhibition of head development 

 progresses from median to lateral regions 

 in pieces undergoing reconstitution. Eyes 

 and ganglia show all degrees of approxi- 

 mation to the median plane even to a single 

 median eye (cyclopia) and a single median 

 ganglionic mass. With a greater degree of 

 inhibition the median tip of the head is 

 reduced, the lateral lobes develop more or 

 less anteriorly, also approximate the me- 

 dian plane, or a single median lobe de- 

 velops. The still more inhibited head is 

 anophthalmic with only a rudiment of 

 ganglion and a single median sensory lobe 

 or none, and finally differential inhibition 

 results in a completely acephalic form. It 

 is a point of particular interest that these 

 differential inhibitions do not specifically 

 eliminate particular organs, but eliminate 

 regions progressively from median to 

 lateral, with whatever parts of cephalic 

 ganglia, eyes, etc., are normally localized 

 in these regions. 



Essentially similar continuous graded 



series of head forms occur with differential 

 inhibitions increasing in degree in the em- 

 bryos of fishes, amphibia, and the chick. 

 With increasing degrees of inhibition, bi- 

 lateral organs of the head, the nasal open- 

 ings, the suckers of the anuran, and the 

 eyes, appear progressively nearer the me- 

 dian plane, develop as fused or single 

 median organs, or fail to appear ; the brain 

 is correspondingly differentially inhibited, 

 and with extreme inhibition acephaly may 

 be approached or attained. To what extent 

 these vertebrate inhibitions are due to dif- 

 ferential inhibition of the inductor under- 

 lying the ectoderm or to inhibition of 

 ectoderm need not concern us here; how- 

 ever, the susceptibility of the inductor does 

 decrease laterally from the median region. 

 The important point is that the inhibitions 

 represent continuous graded series of dif- 

 ferential effects which are not specific for 

 a particular inhibiting agent. 



In the cephalopod embryo a correspond- 

 ing series of head forms with approxima- 

 tion of eyes, cyclopia, anophthalmia, and 

 parallel elimination of other parts of the 

 head progressively from median to lateral 

 regions results from differential inhibition. 

 Although there is nothing that can be 

 called a head in the sea urchin larva, dif- 

 ferential inhibitions of development iden- 

 tical in principle with respect to the 

 gradient pattern present occur with all 

 agents used thus far. In early stages in- 

 hibition decreases from the apical region 

 basipetally; the oral lobe of the pluteus 

 larva, a differentiation of the apical region, 

 is decreased in size, not fully differen- 

 tiated, or may be completely absent while 

 basal regions approach normal. A ven- 

 trodorsal differential, decreasing laterally 

 from the median ventral region is also 

 present. Under natural conditions two 

 arms with calcareous skeletal rods develop 

 diverging at a slightly variable angle right 

 and left from the basal ventral region. 

 With increasing action of inhibiting agents 

 the angle between the arms decreases, they 

 are approximated to the median plane, be- 

 come parallel, develop as a single median 

 arm with double skeleton or entirely single, 



