ON THE PKOCESS OF WASTRULATION IN CHELONIA. ^29 



of the two Jast-named species. MeanwhiJe I have discovered that when 

 C h el o nia caouana deposits its eggs, they are in a far less advanced 

 condition than those of 1'rionyx or Clemmys and thus enable us 

 to elucidate many points in the much discussed process of gastrula- 

 tion in the Amniota. The present contribution embodies the results 

 of my own study on this point and, it is hoped, will throw light on 

 some phases of this vexed question. 



Preparation and Preservation of the Embryos. 



Young embryos were in nearly all cases preserved in Kleinen- 

 berg's picro-sulphuric acid. Very advanced embryos were placed, 

 partly in that fluid, and partly in corrosive sublimate. In removing 

 blastoderms from eggs within one or two days of their deposition, at 

 which age there is not yet any large subgerminal cavity in the yolk, the 

 shell was removed and as much of the white as possible. The whole 

 egg was then placed with the blastoderm uppermost in a deep vessel 

 and covered with picro-suJphuric acid. The spot where the blastoderm 

 was to be found was generally marked with a hair since the thin layer 

 of the white necessarily left over it coagulates in the preserving fluid 

 and hides it entirely from view. Proceeding in this way the pre- 

 serving fluid will be found after three or four hours to have penetrated 

 to the blastoderm and acted on it as also on the upper strata of the 

 yolk. Incisions at right angles were now made with a sharp knife 

 on three sides of the blast(jderm, leaving the fourth side and the two 



corners uncut, as shown in the accompanying diagram. 



It was then found that a little manipulation with forceps 



i)v scalpel easily separates the superficial coagulated white 



from the blastoderm beneath it. If we then cut the 

 corners, the sheet of the white will roll up of itself towards the uncut 

 side, leaving fully exposed the blastoderm which bei)ig already hardened 



