912 



SCIENCE 



[N. S. Vol. XXVII. No. 702 



The heterolateral head movement is a 

 unit in the ' ' action system. ' ' This unit is 

 predetermined regardless of any experience 

 on the part of the individual. The first 

 response to a touch on the tail bud is a 

 head movement and as the whole trunk 

 comes to be flexed the movement still be- 

 gins in the head region and progresses cau- 

 dad. The earliest conduction paths, then, 

 of the lower part of the cord are longi- 

 tudinal and afferent, and the two halves 

 are physiologically distinct during this 

 phase of development. Specimens that are 

 asymmetrical for head stimulation are so 

 for tail stimulation also. 



The phylogenetic Differentiation of the 

 Organs of Smell and Taste: C. Judson 

 Hekeick, The University of Chicago. 

 Physiologists have not yet succeeded in 

 finding a well-defined criterion for the 

 separation of these senses in terms of the 

 character of the stimulus, nor can they 

 be easily distinguished psychologically. 

 Nevertheless, their nervous mechanisms are 

 very different and these anatomical differ- 

 ences seem to have assumed practically 

 their definite form in the very lowest verte- 

 brates. In seeking for the explanation of 

 this differentiation of two chemical senses 

 in low aquatic vertebrates, attention is di- 

 rected to the character of the motor re- 

 sponses as giving the key to the evolution 

 of the very diverse reflex paths for smell 

 and taste. The typical response to stimu- 

 lation of the taste buds within the mouth 

 is a visceral reaction (mastication, swal- 

 lowing, etc.) ; that of olfactory stimuli is a 

 somatic reaction, usually toward or away 

 from the source of the stimulus. A quite 

 different set of nervous pathways is in- 

 volved in the latter case from that of the 

 former. Assuming that smell and taste 

 have evolved from a common type of un- 

 differentiated chemical sense, this differ- 

 ence in mode of reaction to chemical stim- 



uli within and without the mouth is ade- 

 quate to call forth in the course of further 

 evolution the structural differences which 

 we find between these organs in higher ver- 

 tebrates. 



The Mesencephalic Boot of the Trigeminus 

 in BeptUes and Mammals: J. B. John- 

 ston, University of Minnesota. 

 This btindle of fibers has long been 

 known in man and mammals and described 

 as the descending or cerebral motor root 

 of the trigeminus. The writer has de- 

 scribed it in selachians, ganoids and anur- 

 ans as a part of the sensory root. The 

 same is now found to be true in reptiles 

 (turtle, snake) and in mammals (mole, cat, 

 white rat, pig embryo and human embryo 

 of 15.5 mm.). In the adult mammals 

 studied the bundle in question is made up 

 of two parts. One part arises from the 

 spinal trigeminal tract at about the level of 

 the facial nerve, runs dorsalward through 

 the substantia gelatinosa, turns cephalad 

 between the gelatinosa and the vestibular 

 centers, passes over the dorso-caudal sur- 

 face of the motor nucleus of the trigem- 

 inus, and gains the internal face of the 

 brachium conjunctivum near the fourth 

 ventricle. Here it is joined by the second 

 part, which comes up from the sensory tri- 

 geminal root among the trigeminal fibers 

 to the cerebellinn. The combined bundle 

 has on its mesal and ventral side a column 

 of large and small cells which is closely 

 related to (merges with) the vestibular 

 nuclei. The bundle and column of cells 

 continue forward into the mesencephalon 

 in the well-known manner. 



In the human embryo studied (and pig 

 embryos also) the relations of the sensory 

 and motor roots at their connection with 

 the brain are such that there seems no pos- 

 sibility of confusing the two. The bundle 

 under consideration is made up in the same 

 way as above described and is further 



