ORGANS IN THE EMBRYO OF THE FOWL. 



103 



of the rotation would much exceed that of c-d (Fig. 4, //) ; and 

 if during this process there were actual independent growth of 

 a-b and c-d, the latter would of necessity be thrown into folds, 

 but not the former. Finally, if the amniotic folds were forming 

 at the same time (as is actually the case) the right one would 

 inevitably be thrown into secondary folds by the approximation 

 of points c and d (Fig. 4, C). 



Study of the fusion of the amniotic folds in actual section 

 shows (i) that the line of fusion of the opposite amniotic limbs 

 is over the dorsal surface of the embryo only so long as the latter 

 lies flat on tJie yolk, and does not follow the turning of the embryo 

 on to (usually) its left side ; the consequence is that after rotation 

 of the embryo the line of fusion lies over the upper (right) side 

 of the embryo, often opposite the horizontal level of the intestine 



/ 



FIG. 5. Transverse section of an embryo of about 48 hours (Duval) showing the 

 position of the ectamnion on the right and left sides. University of Chicago Embry- 

 ological Collection, No. 689. e.a., ectamnion ; /., left ; s.f , secondary fold of am- 

 nion on the right side. The great differences in the thickness of the amnion of the 

 right and left sides should be noted. 



(Fig. 6). Thus one fold of the amnion passes all the way from 

 the under side over the back of the embryo and around on the 

 other side to the line of fusion, and thus is several times as long 

 as the opposite limb. (2) Moreover, the amniotic fold of the 

 right side is invariably thicker than that of the left side, and is 

 always thrown into secondary folds at the place of turning (Fig. 

 5 and Fig. 6). These conditions are satisfactorily explained, as 

 noted above, by the mere turning of the embryo on its side. 



One must therefore distinguish in the upper limb of the am- 

 nion two kinds of folds: (i) The ordinary amniotic fold induced 

 by the fusion of the right and left rudiments and (2) secondary 



