Tlie Roactions of the Vertebrate Embryo to Stimulation etc. 539 



rally be abolished or inhibited by the action of cocaiu — a few 

 dro})S of a 2 o solution added to each 50 ce. of the fluid in which 

 the embryo swims are quite sufficieut for this purpose. Chloretone 

 (acetonchloroform) seems to have a similar eftect, but does not ap- 

 parently weaken the heart's action to the same extent as does 

 Cocain. When the movements begin to assume the pronounced cha- 

 racteristics of the preagonal stage neither the former nor the latter 

 seera to diminish their intensity. 



The exact moment at which these movements begin unquestio- 

 nably varies in diflferent embryos. The same holds true for all 

 muscular contractions includiug those of the heart. Generally in 

 embryos {Pristiurus, ScylUum, Torpedo) which have attained a length 

 of 4 mm. thej are so well developed as to be easily recognized. 



A ScyUiu)?i emhvyo, length 6 mm., lying in a small glass vessel 

 containing salt solution was placed upon the stage of a microscope 

 and the reactions studied with the aid of Zeiss ocular 2 — obj. a3. 

 The bodily movements which at this period are very well marked 

 occurred in distinct groups, three or tour rapidly succeeding each 

 other were followed by a pause. The outward swing of the head 

 and tail seemed to be due to a vigorous impulsion or rather an ac- 

 tive muscular contraction taking place in the middle third of the 

 embryo. The return excursion to the middle line however, was 

 slower and gave the impression of being merely a passive move- 

 ment, similar to the manner in which an india-rubber band flies 

 back into place after the tension has been relaxed. 



In an embryo of the same species, length 7 mm., the character 

 of the return swing resembled that of the outward excursion, and 

 after prolonged Observation the inference that it was an active, and 

 not merely a passive movement, seemed to be justifled. In the 

 earliest stages when these movements occur it is uoticeable that the 

 excursions of the head are greater than those of the tail. This 

 may be in part the result of the attachment of the embryo and in 

 part referable to the fact that the myotome, which is more fully de- 

 veloped in the cranial than it is in the caudal part of the body is 

 beginning to function. If a needle is carefully slipped under the 

 head or the tail of an embryo of 8 mm. — 9 mm. in length and one 

 end is gently raised, the length of the excursions of the other are 

 changed without being completely inhibited. 



In Pristiurus or Scyllium embryos (length 10 mm.) if the head 

 Ì8 allowed to swing only in one direction, for example to the right, 



