ON THE FOETAL MEMBKANES OF CHELOXIA. 3 



l)l:iwr(jpor(' is \ery distinct. At tiie posterior end of the eniljryonio 

 region, there is, in the specimen figured, îi h)vv semilunnr fold bound- 

 ing the embryo from behind. It gives one the impression of its 

 being the posterior fold of the amnion. Such folds are not, however, 

 found by any means in all the embryos, and even when present, are 

 not always of the same figure and distinctness as in the figure. As 

 the subsequent history shows, these inconstant folds ut the posterior 

 end of tlie eml)ryo take no share wliatever in the formation of the 

 permanent amniotic sac. 



Hound the head-end of tlie embryo, there is an irregularly semi- 

 circular transj)arent area of the blastoderm. In this area, there is 

 usually an opaque line also semicircular and concentric with the 

 cephalic grooxe (Fig. 1). Kound the postei-ior end of the embryo, 

 and along its sides, there is a brcjad h(jrse-sh<^e shaped opac(ue streak 

 which is caused by the abundant accumulation «jf yolk-granules — 

 the germinal wall. The mesoblast, at this stage, extends into the 

 head of the embry«) [)r<jper, but anteriorly, laterally, and postei'iorly 

 the opaque liorse-slioe shaped streak mai'ks the limit of its extent. 

 Hence the transparent area in front of the embryo is as yet free from 

 the mesoblast . 



In Fig. n.S, IM. \ll. (See also Diag. 1. IM. X.), a median longi- 

 tudinal section of the head end of the embryo is represented. It is 

 evident from this section that the deep horse-shoe shaped groove at 

 the anterior erid (a. /../.) is bounded posteriorly by the head (H. F.) of 

 the embryo, while its anterior wall forms the first rudiment of the 

 anterior fold of the :tmnion (Anin). The anini(jn is thus laid in the 

 region into wliidi the mesoblast has not yet found its way, and 

 ihcTefore. of nercssitN , consists at lirst only of the <'|)iblas( and hypo- 

 blast. In Fig. 59 (Fl. VII.), a transverse section of the same region 

 is represented. From this and Fig. 58 (PI. VFT.). the characters 



