k80 GLOSSARY 



YOI£ PLUG - large yolk cells which are too large and sluggish to be Innnadlately Incorporated in the floor 

 of the archenteron, hence are found protruding slightly from the mouth of the blastopore to form a 

 plug vhich la distinct hy color from the surrounding pigmented marginal zone (amphihla). Term used 

 as identification of a late stage in gastrulatlon. 



ZONE, MAEGINAL - presumptive chorda-mesodermal-endodermal complex at the Junction of the roof and the 

 floor of the early gastrula. Syn., germ ring. 



HOTE : A glossary of some 350 terms in morphological embryology may be found 

 In the author's "Laboratory Manual of Vertebrate Embryology". 



"On the other handt some very fundamental advances are, upon critical 

 examination, found to rest upon the establishment of a j udic ious defini- 

 tion, A notable instance of this is the enunc iat ion of the pr inc iple of 

 the survival of the fittest, which is essent ially of the nature of a defini- 

 tion, since the fit is that which survive s . " 



Lotka 1925 



