382 E. L. CLARK AND E. R. CLARK 



Selachian embryos of 3,5 to 4 mm., according to Paton, jerky 

 lateral movements begin. These are rhythmic in character: sev- 

 eral movements occm*, followed by a pause of several seconds 

 and then the movements begin again. They do not appear to 

 be affected by mechanical stimulation of the embryo, although 

 they are increased by squeezing the yolk sac. The rhythm of 

 the movements is altered by heat, by brilliant light, and by 

 interfering with the normal swing of the head from side to side. 

 In all of these cases, the movements become more violent. In 

 the case of approaching death, the contractions become tonic. 

 Fish embryos appear to resemble chicks in the periodic char cter 

 of their early movements and in their failure to respond to 

 mechanical stimuli. 



In Mammalia, Preyer (pp. 586-595) studied the movements 

 in guinea-pigs. The earliest movements were seen in a 'fourth 

 week' embryo of 16 mm. which ''bewegte den Rumpf in situ 

 stark," In this embryo, he notes ''Zehen noch nicht getrennt," 

 In all embryos of successive weeks, movements were seen, in- 

 creasing in intensity. In the 16 mm, embryo electrical stimu- 

 lation over the spinal cord caused muscle contractions, and in 

 26.4 mm. embryos peripheral stimulation, both electrical and 

 mechanical, led to reflex movements. 



That periodic muscular movements, beginning at least as early as 

 the fifth day of incubation and continuing with increasing intensity 

 throughout the remainder of incubation, take place in developing chicks 

 (and that movements occur in mammalian embrj^os as well, begin- 

 ning at least as early as 16 mm. in guinea pigs — Preyer) is a fact of 

 much significance in the study of the phsyiology of the embryo and of 

 the mechanical factors concerned in development, especially of mus- 

 cles, bones and joints — a fact which has been frequently overlooked. 



Thus Thoma, in his study of bone growth, takes up the mechanical 

 factors acting on cartilage at the time of beginning ossification. On 

 page 350 he says: "Als Ursache der Belastimg kann demgemass im 

 wesenthchen nur die Spannung der Hautdecken und der Muskulatur." 

 No mention is made of the tension caused by contraction of the muscles. 



Again R. Schmidt, p. 95, in discussing the ossification of bones in 

 embryos, concludes "Da die Grundzlige der Architektur sich ausbilden, 

 ehe noch die Muskelelerfiente kontraktil sind (Hencke, Bernay, u. a) 

 kann die Ursache nicht unmittelbar in Gebrauch zu suchen sein." 

 Herbst (p. 7), after quoting the above, adds "so schliesst Schmidt ganz 

 richtig." The tension produced by muscle contractions, left out of 



