294 OUTLINES OF CHORDATE DEVELOPMENT 



of course, the somatic portion of the blastoderm, and at this 

 time has no direct connection with any other structure. The 

 space between amnion and chorion is exoccelom. This cavity 

 and the amnionic cavity are filled with a watery fluid. Later 

 on, as we shall see, this space is largely occupied by the allan- 

 tois, which comes into intimate connection with the inner 

 surface of the chorion. 



ch 



FIG. 119. Transverse section of chick embryo with thirty-five pairs of 

 somites (about seventy-two hours), passing through the region of the twenty- 

 third somite. After Lillie. a, Amnion; ac, amnionic cavity; ao, dorsal aortse; 

 c, embryonic ccelom; ch, chorion; d, dermatome; ebc, exocoelom; g, rudiment of 

 spinal ganglion; m, mesonephric tubules; my, myotome; p, posterior cardinal 

 vein; s, sclerotome; sa, sero-amnionic connection; sc, subcardinal vein; so, 

 somatic mesoderm; sp, splanchnic mesoderm; v, vitelline artery; TF.Wolffian duct. 



One important fact has not been mentioned thus far. In 

 the region where the amnionic folds finally close together over 

 the embryo, the amnion and chorion remain firmly united 

 (Fig. 119). This forms the sero-amnionic fusion, which affects 

 in an important way the later arrangement of these mem- 

 branes, as well as the disposition of the allantois, when it 

 extends into this region (Figs. 115, 116). 



The regular arrangement of the lateral folds of the amnion is 

 disturbed somewhat by the twisting of the embryo, which is 

 going on as these folds are becoming elevated and fused. As 

 the embryo turns upon its left side the amnionic folds do not 

 similarly alter their morphological relation to what was 



