828 Comparative Animal Physiology 



ming and sensitive over the whole body. Young (2mm.) tadpoles of Rana 

 show longer lasting responses to tactile stimulation than do 4 mm. tadpoles; 

 lesion to the midbrain, however, removes an inhibiting influence in the older 

 tadpoles and leads to more persistent responses."*-*' 



The development of behavior in birds and mammals shows great species 

 variability and differs from behavior in Amhlystoma in that there is no move- 

 ments of the trunk before limbs appear. The literature on various animals 

 has been summarized: rats,^'^ sheep,=^^- •^- guinea pigs and rats,^^ cats,-*-^"* and 

 general summary.^^ 



In general, the latent period and contraction time of embryonic muscle 

 are long. There is an initial pre-motile stage when local contractions are elic- 

 ited by direct stimulation of some muscles. The first spontaneous movements, 

 usually in the neck, are of uncertain origin but appear to be reflexes.^^ The 

 earliest definite responses to mechanical stimulation are gross reflexes of the 

 neck (head movement), and then of the forelimbs. In the rat, limb move- 

 ments occur first in conjunction with movements of the trunk; in the cat, 

 sheep, and guinea pig they appear to be independent. However, these limb 

 movements are gross but integrated, and separate segmental movements de- 

 velop later. Movement appears relatively earlier in the rat, while the sheep 

 starts movement at a later stage.*^^ The segregation of reflexes depends on 

 interneurone development and the appearance of inhibitory mechanisms. 



Development of cephalad inhibition of lower motor centers is well shown 

 in the sheep fetus.^^-^^ Movements are at first (at 40 days gestation) jerky, 

 later smooth and sustained, indicating repetitive nerve cell discharge; still 

 later (at 60 days gestation), the fetus is inert to stimulation on the snout. 

 Anoxia inhibits the movements acquired last; for example, it reverses the 

 response when an animal is in the inert stage, resulting first in sustained and 

 then in jerky movements. Also brain transections show that the jerky move- 

 ments are mediated by bulbospinal systems; these are converted to sustained 

 movement by the midbrain, and inhibited by cephalad portions of the brain. 

 In newborn monkeys tonic innervation of muscles and grasp reflexes are 

 dominant, then in a spastic stage limbs resist passive movement; finally dis- 

 crete use of muscles appears. If in the adult a premotor area of the cerebral 

 cortex (area 6) is removed the grasp reflexes seen in the newborn return. ^^^ 



In some species of animals, then, the first reflex responses are generalized, 

 in others they are somewhat localized, but in both types the movements are 

 integrated from the start, and restricted movements become individuated later. 

 Behavior development follows morphological gradients and cephalic domin- 

 ance is important in the sequence of responses. 



GROSS FUNCTIONAL EVOLUTION OF THE BRAIN 



Neuroanatomists, particularly under the influence of Ariens-Kappers,^'^ 

 picture the anterior enlargement of the neural tube phylogenetically and 

 the expansion of the medulla as a response to the increase in sense organs 

 in the head. The medulla shows greater development of sensory roots than 

 the spinal cord. There is a progressive phylogenetic reduction in the number 

 of branchial arches; there are 36 in Am-phioxus, 8 in Petromyzon, and 5 in 

 the sharks. Many changes in the sensory components occur from group to 

 group. For example, in fish the numbers of visceral sensory and gustatory 



