THE EARLY DEVELOPMENT OF AMPHIOXUS. 593 



Vertebrate developmentwhere,the yolkbeingevenlydistributed, 

 its disturbing influence is negligible ; and when one recollects 

 the weary controversies which have been waged round the 

 meaning to be attached to the gastrulation and formation of 

 the layers in the heavily yolked eggs of the higher Verte- 

 brates, one must feel that a knowledge of the development of 

 Araphioxus alone could bring these questions to a definite issue. 



The results I have arrived at differ considerably from any 

 obtained hitherto, and in claiming to have penetrated more 

 deeply into the developmental processes than Hatschek or 

 Kowalevsky I rely entirely on the higher grade of perfection 

 which methods of dealing with small organisms have reached 

 in the meantime. 



All the embryos were embedded in the ordinary way in cel- 

 loidin ; but after hardening the celloidin with chloroform I 

 adopted a plan of clearing on which really the whole success 

 of the work depended. This was as follows : — The hardened 

 celloidin was immersed for a minute or so in absolute alcohol, 

 in order to remove any traces of moisture which might be 

 present in the chloroform ; it was then placed in cedar oil and 

 left for a night in a warm place (the dish containing the cedar 

 oil being placed on the top of the thermostat). In the morn- 

 ing the celloidin had become so transparent as to be almost 

 invisible when looked at in the cedar oil. Little pieces of the 

 block containing the embryos could then be cut out and 

 examined with ease under a low power, and their exact orien- 

 tation determined. They were then embedded in paraffin, and 

 cut into series of sections from 4 to 5 ju thick, and stained on 

 the slide. 



The material at my disposal was preserved in a variety of 

 ways, but except for the earliest stages my results are based 

 only on specimens preserved in osmic acid. I wish to lay 

 particular emphasis on this, as should any zoologist feel 

 inclined to work over the same ground with a view of testing 

 my results, and use such fixing reagents as corrosive subli- 

 mate or picro-sulphuric acid, he is foredoomed to failure. 

 After such fluids the gut becomes swollen and the body-walls 



