ORGAN'S IN THE EMBRYO OF THE FOWL. IO$ 



of rotation of the embryo, and are coincident with the closure 

 of the amnion (Fig. 3) ; they persist until the body-wall is com- 

 pleted behind the entire heart. They are not, in my opinion, 

 exclusively folds of the amnion, but extensions of the body-wall 

 for enclosure of the region of the heart and liver. The direct 

 cause of their formation is, however, the rotation of the embryo 

 with extreme growth of the body-wall contiguous to the amnion, 

 and fixation of the outer end of this limb of the amnion by the 

 amniotic suture. 



Elsewhere the effect of the twisting of the embryo is rapidly 

 compensated so that the secondary folds of the right half of the 

 amnion do not persist long except in the region of the allantois, 

 where slight inconstant secondary folds may continue longer. 



B. Experimental. 



I. Experiments on the Head-fold of tlic Ainnion. 



Experiment No. 57. 



Age of the embryo at the time of operation, 33 hours 1 

 (Duval). 



Operation. The blastoderm was cauterized lateral to the right 

 optic vesicle with a needle (Fig. 7) so as to make a large open- 

 ing. At the time of the operation only the most anterior horse- 

 shoe-shaped segment of the ectamnion was present (cf. Fig. i), 

 and this was destroyed only on the right side of the embryo. On 

 the left side, therefore, the amniotic fold was free to form to the 

 extent that it is independent of the opposite fold. The right optic 

 vesicle was slightly injured, as the results of the experiments 

 show. In opening the egg for the operation, the blastoderm was 



1 In describing the various experiments, the age of the embryo at the time of the 

 operation will not be given as the actual number of hours in the incubator, because 

 the variations in point of actual development after the same period of incubation are 

 so extreme. It is not possible either to make accurate measurements of the living 

 embryo or to determine the number of somites present, on account of the loss of time 

 and danger of exposure of the embryo. A rough sketch of the embryo was always 

 made at the time of the operation, and this is sufficient to identify it with the various 

 staaes fio-ured in Duval' s atlas. The age is based on this identification. Thus the 



r- 



given age at the time of operation in these experiments represents a certain dehmte 

 stage of development. On the other hand, the length of time that elapsed from the 

 experiment to the time of reopening the egg is always given literally. 



