The Reactions of the Vertebrate Embryo to Stimulation etc. 543 



tliat does exist between the sensitiveness of liead and body is, imtil 

 a comparatively late stage in the life of the embryo, cousiderable. 

 Tue epidermal layer of the former may be gently rubbed or })ricked 

 without any visible response, a condition that is in marked contrast 

 to the series of reflex activities initiated b}' stimulating the latter. 



The eye seems to be quite insensible to taetile Stimuli in ali 

 the stages examined. 



In Pristiunis, Scyllium and Torpedo in contrast to Trout and 

 Salmon the spontaneous movements of the lower jaw are absent. 

 In the two latter they may be easily seen in embryos of 16 mm. or 

 17 mm. while the gills move rhytlimically and rapidly as early as 

 13 mm. 



The difficulty of determining the efifect of light as a Stimulus 

 is very great. In a number of instances I bave observed that if 

 Pristiurus embryos (12 — 14 mm.) after removal from the e,^^ case 

 were placed in a glass dish containing 3^ salt solution and bright 

 sun light was then suddenly thrown upon them by means of a mirror 

 that either one of two thiugs happened. If the embryo prior to this 

 Stimulation was resting quietly on the bottom of the dish an attempt 

 was at once made to swim ; but if movements were already being 

 executed at the moment when the incident light Stimulus feil upon 

 the body a change in their rhythm and number took place. It was 

 not determined accurately whether one part of the surface of the 

 body was more sensitive to the action of light rays than was another. 

 This general appare nt sensitiveness to the action of light rays may 

 really be due in part at least to the beat rays. Without further study 

 it would not be justifiable to compare these i)rimitive reactions to 

 those which develope later on and are undoubtedly the result of the 

 functional activity of the retina. 



The prompt and unmistakable responses to thermal Stimuli appear 

 at an early stage — for example energetic wrigglings occur in trout 

 (14 mm.) when touched with a warm needle, and the same is equally 

 true of the selachians. 



The study of the cardiac movements was particularly inter- 

 esting, for it was observed that with the exercise of care in removal 

 of embryo from the shell that the heart was generally seen to be 

 beating regularly and with cousiderable force in selachians only 

 5 mm. long. In two or three instances I bave noted regulär un- 

 mistakable cardiac activity in embryos measuring 4 mm. « 



Mittheilnngen a. d. Zool. Station zn Neapel. Bd. 18. 35 



