FATE OF BLASTOPORE, ETC., IX CHELOXIA. 3 J 



is covered over by various structures which render it no longer 

 possible to be seen in a surface-view, even if present. 



Summary. 



The changes which I liave described as observable in the surface- 

 views of the embryos of the three species may be summed uj3 as 

 follows : — 



The mass of cells 7vhich sticlcs out hetireen the limhs of the horse-shoe 

 shaped blastopore — i.e., the yolk-plug — is emhraced between the posterior 

 ends of the luedidlanj folds, udien these are formed, and is lifted and 

 compressed lateralhj bij their coming together. 



It then apparently shifts its position had- wards. 



In Chelcnia and Clemmys, the yolk-jilug in thus vioring baclcwards, 

 leaves, so to speak, in its track a groove n-liich therefore necessarily joins the 

 hindmost portion of the medullary canal villi the groove surrounding the 

 yolk-plug. Tins is the primitive groove. 



In Trionyx, sucli a groove is not visible on the surface. 



TJie " Endundst " rises up as the tail of tlie cmbrijo, and tlie line 

 of upheaval crosses the primitive groove (^u^hen such is present) at right 

 angles and midway between the initial -position of the yolk-plug and its 

 final backward position. 



The yolk-plug therefore is not included in the tail and lies some 

 distance behind it. 



B. THE SECTIONS. 



In giving an account of the internal developmental processes of 

 which the chano-es described in the precedinç;' section mav be said to 

 be the surface expressions, the most logical way would be to give 

 parallel series of sections of different stages in each of the three species 

 that farmed the objects of my research. Investigators of similar 



