506 



MANUAL OF ELEMENTARY ZOOLOGY 



addition of new cells cut off from the yolk. The 

 blastopore is represented by a longitudinal strip in the 

 hinder part of the blastoderm, where the epiblast is thicker 

 than elsewhere and remains longer in continuity with the 

 lower layer. This is the primitive streak, and is marked by 

 a longitudinal primitive groove (Fig. 384, 3). The formation 

 of the organs of the chick cannot be followed here. In 

 broad outline it takes place in the same way as in the frog. 

 Something must he said, however, 

 about the folding off of the embryo 

 and the formation of the embryonic 

 membranes which are found in rep- 

 tiles, birds, and mammals. As de- 

 velopment proceeds, furrows appear 

 in the blastoderm of the area pel- 

 lucida around a region which is 

 forming the embryo, beginning in 

 front and behind as head and tail 

 folds and afterwards joining at the 

 sides. These deepen, and pinch off 

 the little embryo proper from the 

 yolk, which is enclosed in a yolk sac 

 formed by the growth round it of the 

 blastoderm. As the embryo grows and 

 absorbs the yolk it becomes larger 

 than the yolk sac. The amnion is a 

 peculiar membrane which envelops 

 the embryo and arises in the following 

 way. At a time when the splitting of 



Fig. 385. — The origin of amnion and 

 allantois. — After Balfour. 



1. Rise of amniotic folds (a.f.) around embryo (e) ; 



p.p., pleuro-peritoneal cavity or ccelom ; 



y.i yolk. 

 a. Further growth of amniotic folds {a./.) over 



embryo and_ around yolk. 



3. Fusion of amniotic folds above embryo; «./., 

 amnion proper ; s.ctn., false amnion or sub- 

 zonal membrane ; jk.j., yolk sac. 



4. Outgrowth of allantois (al.) ; amniotic cavity 

 (a.r.); /(., head end ; t., tail end. 



5. Complete enclosure and reduction of yolk sac 

 (y.s.) ; s.s.w., subzonal membrane ; a.fl., 

 amnion proper; aL, allantois; #., gut of 

 embryo. 



S.z.rn 



