538 Stewart Paton 



cognizable and possess certain definite and distinguishing characte- 

 ristics in those of 4 mm. in length. In several instances at tlie 

 earlier stage (3,5 mm.) it has been noted that about 1 mm. from the 

 head end of embryo there is a point of flexion at which jerky la- 

 teral movements take place. Oceasionally the tail end, but to a 

 less extent, is also deflected laterally at the same instant that the 

 anterior part Swings out from the median line. Several of these 

 movements more or less rhythmic in eharacter oeeur at iutervals of 

 one or two seeonds and are foUowed by a pause of several seeonds, 

 and then the cycle begins again. Although the entire head, and as 

 has been said, sometimes the tail, swing laterally so that at 

 the end of the outward excursion the entire embryo on the side 

 towards which the movement takes place forms a concave curve, it 

 is probable that only the middle third of the body is actively cou- 

 cerned in the movement, the two ends playing only a passive part 

 in the swing. The pivotal point at which this flexion seems to 

 occur is practically just above the point of attachment of the embryo 

 to the egg, at a point where the myotomes develope most rapidly, 

 and where neurofibrils are found as early as the stage represented 

 by embryos of 5 mm. — 6 mm. in length, appearing in the syncytium 

 or Plasmodium formed by the primitive myoblasts or their ante- 

 cedents. 



An exceedingly important point in connection with these primi- 

 tive movements is that, if each individuai excursion of the body is 

 compared with those taking place at a tinie when the reactions re- 

 present the active participation of the skeletal muscles, it becomes 

 exceedingly difficult to determiue the individuai characteristics which 

 distinguish the earlier from the later forms. The differences are 

 merely those of degree, but not of kind. If the temperature of the 

 Salt solution is the same as that of the water from which the egg 

 was taken these movements, even in small embryos, may continue 

 for hours. The signs of approaching death are a loss of the rhyth- 

 mic eharacter of the movements which soon become almost conti- 

 nuous. At first the length of excursion is greater, giviog the Im- 

 pression of hyper-abduction and hyper-adduction, at the same time 

 becoming more abrupt and jerky in eharacter. The ab- and adduc- 

 tion occurring in embryos between 3,5 mm. and 5 mm. and which 

 under normal conditions are the only ones observed, in this preagonal 

 stage are frequently accompanied by a dorsal flexion which at times 

 becomes tonic. The first of these spasmodic contraetions may gene- 



