The Reactions of the Vertebrate Embryo to Stimulation etc. 537 



Pristiurtis, Scyllium, Torpedo and Lacerta embryos are more complete 

 than are tliose made upon the other species, the attention will be 

 direeted chiefly to the eonditions existing in the three types of the 

 Elasmobranchs, and the single representative of the Lacertidae. 



There are two reasons why the results obtained have not been 

 equally satisfactory in regard to ali species studied. In the first 

 place the ìnvestigatious made upon the Elasmobranchs and Lacerta 

 were begun at a time when I had profited greatly by the expe- 

 rience gaiued by studying other embryos. In the second place the 

 case with which the Elasmobranchs may,. at a very early period 

 in their development be removed from the case in which they are 

 enclosed, and kept alive for considerable periods of time either in 

 normal salt solution or inaSYjVo solution, which approximates the 

 specific gravity of sea water, was an important factor in facilitating 

 Observation. 



In Lacerta, on account of the extreme difficulty of removing the 

 embryo uninjured from the shell the importance of the physiological 

 observations was much less than those connected with the study of the 

 tissues. In Selachians, in order to reduce the source of possible 

 error to a minimum, the attempt was frequently made to study the 

 character, rhythm, and rapidity of the movements before the egg-case 

 was opened; the egg being placed for this purpose in a glass 

 vessel filled with sea-water of the same temperature as that con- 

 tained in the aquarium from which the egg had been taken. Un- 

 fortunately, owing to the thickness of the egg capsule, it is only 

 possible by this procedure to determine more or less roughly the 

 character of the movements. Even with the exercise of the greatest 

 caution in the removal of the embryo from the capsule there is at 

 first nearly always a well marked alteration in the rhythm of the 

 movements, due to a variety of causes, depending upon the physical 

 and chemical dififerences that exist between the fluid contained within 

 the capsule and that to which the embryo finds itself suddenly trans- 

 ferred. The difficulties are greatly iucreased if the attempt is made 

 to remove the young Trout or Salmon embryo from its shell on 

 account of the relative large size of the sac — and therefore 

 these species are far less adapted to experiments of this nature than 

 are the Selachians. 



The first movements that occur in Pristiurus embryos in any 

 way suggesting the possibility of muscular activity have been occa- 

 sionally seen in embryos of 3,5 mm., but as a rule are easily re- 



