PHYSIOLOCilCAL INTERPREIATIONS 71 



in terms of current action. The elaboration of the analytic apparatus 

 on the sensory side is carried only so far as is requisite to facilitate 

 responses to any combination of sensory stimuli in patterns deter- 

 mined by the appropriate use of such motor equipment as the animal 

 possesses. In species with simpler action systems the central analytic 

 apparatus is less elaborate; in species endowed with more compli- 

 cated motor organs the central architecture is more elaborate. In all 

 species the peripheral sensory equipment determines the architec- 

 tural plan of the primary centers of the sensory zone; internally of 

 this level the details of the plan are shaped by two additional factors : 

 first, the motor equipment available and, second, the amount and 

 quality of the apparatus of correlation and integration required for 

 the most efficient use of such sensory excitations as the animal ex- 

 periences. The cross-connections between the sensory and motor 

 zones are quite direct and simple in early embryological stages, so 

 arranged as to provide uniform stereotyped responses to oft recurring 

 situations. But as development advances these connections become 

 more and more complicated, an intermediate apparatus of correlation 

 is interpolated, and, correlated with this, the behavior becomes more 

 diversified and unpredictable. 



In the sequence of development of behavior patterns this change 

 can be accurately dated. For instance, in Amblystoma between the 

 early swimming and early feeding stages, at about Harrison's stage 

 40, the swimming movements, which in younger stages are perfectly 

 co-ordinated by the bulbo-spinal central apparatus alone, lose this 

 autonomy, and participation of the midbrain is essential for the 

 maintenance of efficient swimming, as was mentioned on page 62 in 

 describing experiments by Detwiler ('45, '46). It is during this period 

 that tecto-bulbar and tecto-spinal connections of essentially adult 

 pattern are established ('39, p. 112). In human fetal development 

 there is a similar critical period at about 14 weeks of menstrual age 

 (Hooker, '44, p. 29). At this time the upper levels of the cerebrum 

 acquire functional connections with the lower brain stem, and the 

 behavior shows a corresponding change. "The fetus is no longer 

 marionette-like or mechanical in the character of its movements, 

 which are now graceful and fluid, as they are in the new-born." 



Synthesis and integration may be effected in various ways. The 

 most evident nervous structures employed here are the internuclear 

 tracts which form a complicated web of conductors, which inter- 

 connect the analytic units with one another so that the entire com- 



