TOO 



FRANK R. LILLIE. 



then that behind the tail there is actually a new starting-point for 

 the ectamnion as well as the amniotic folds. The primary posi- 

 tion of the ectamnion is near the boundary of the pellucid area ; 

 towards the posterior end it bends in very sharply, nearly joining 

 the body wall proper, and terminating in the posterior rudiment. 



FIG. 3. Embryo of chick with 21 mesoblastic somites. University of Chicago 

 Embryological Collection, No. 99. e.a., ectamnion; s.jf., secondary folds of the 

 amnion on the right side. The dotted line continuing e.a. represents the continua- 

 tion of the ectamnion beyond the region of folding The dotted area at the angle of 

 the folds represents the ectodermal sero-amniotic connection of Hirota. 



Origin of the Tail-Fold. - - The tail-fold proper arises from an 

 ectodermal thickening lying in a depression just beneath the 

 rudimentary tail-bud. The depression is caused by the enlarge- 

 ment of the body cavity on each side of the middle line. These 

 enlargements may be called the amriio-allantoic enlargements, as 

 they are associated with the formation of the allantois. I would 

 venture the hypothesis that the existence of a separate tail-fold of 

 the amnion is associated with the time of development of the al- 

 lantois, which is represented in the embryo under consideration 

 (i) by a shallow entodermal evagination and (2) a mass of meso- 

 blast. 



