Nervous Systems 827 



non.^^^ As the brain gains importance the spinal cord is less capable of inde- 

 pendent control of locomotion. 



BEHAVIOR OF EMBRYOS 



The sequence of development of behavior, both "spontaneous" move- 

 ments and responses to stimuli, can be correlated with the development of 

 sensory, central nervous, and effector structures. Usually the necessary struc- 

 tures are present well before a given behavior pattern is manifest. In those 

 animals in which behavior development has been studied, several gradients 

 exist, the primary gradient being backward from the cephalad end, and a 

 secondary gradient being laterally from proximal to distal effectors. 



The extensive literature on embryonic behavior emphasizes a contro- 

 versy whether specific reflexes appear first and are later integrated, or 

 whether an integrated pattern precedes the reflexes which individuate out 

 from it. Diverging opinions result from striking differences in the speed of 

 emergence of adult behavior patterns in different species and in the rather 

 futile attempt to compare those embryos which move first without appen- 

 dages with those whose first movement is of appendages. The important 

 generalization emerges, however, that reflex behavior is integrated from the 

 beginning. 



The only invertebrate in which the pattern of behavior has been correlated 

 with histological development is the earthworm, Eisenia foetida.^-^ The first 

 motile stage is a ciliated gastrula, the second is a stage of contractions local- 

 ized around the stomodaeum while the embryo is swallowing quantities of 

 albumin. Next the local contractions lead to peristaltic waves which, when 

 unequal, result in rolling of the body; at this time mechanical stimuli elicit 

 localized contractions, at first antero-ventrally, later laterally and posteriorly, 

 all of these are myogenic responses. Then, when nervous mechanisms are 

 developed, regular flexion of the head begins and the anterior end turns 

 away from a point of stimulation. Head extension and flexion initiate the 

 peristalsis of crawling, and withdrawal from stimuli develops backward un- 

 til the whole animal is sensitive. The worm is capable of burrowing before it 

 hatches. In LimuUis embryos-^^^ kicking of the legs begins soon after the ap- 

 pearance of abdominal respiratory movements, and the trilobite larvae are 

 able to swim well before they hatch. 



In extensive studies on behavior in embryos of Amhlystoma, CoghilP^ 

 identified the following stages: a non-motile stage when the muscles of the 

 somites contract in response to direct stimulation, a stage of simple flexure of 

 the body in response to tactile stimulation, spontaneous bending into a coil 

 and uncoiling, an S-stage, and finally a stage when the S-contractions are 

 executed to effect locomotion. Motor and sensory neurones are present in 

 the non-motile stage, but no bipolar commissural neurones are present until 

 the coil stage. All limb and gill movements appear first in conjunction with 

 trunk movements. Parts of limbs move first with the entire limb and later 

 independently. In general, the first movements are of gross regions, the total 

 behavior complex appears early, and simple reflexes are individuated frpm 

 it. The general pattern in anuran embryos is similar to that in Amhlysto- 

 ma,'^*'^ but there are species differences in rate of development. Bufo is pre- 

 motile at hatching; Acris and Pseudacris, like Amhlystoma, are free-swim- 



