Pearl, Reactions of Linailns. 145 



chelae catch at points lying on the anterior ' hemisphere of 

 the "vicarious chorion." Evidently then if the legs are ex- 

 tended, or in other words, the chelae are strongly pushed away 

 from the bod)', when they are caught in the "vic^irious chorion," 

 a movement of the embryo or of the "vicarious chorion" will 

 be caused. Which of the two shall move, will depend on the 

 circumstances. If the "vicarious chorion" is held in any way, 

 the embryo is moved within it, in the following manner. The 

 anterior edge of the cephalothorax is pushed more and more to- 

 wards the lowest part of the hollow sphere in which the embryo 

 lies, until finally the whole embryo is nearly or quite in a verti- 

 cal position, resting on the anterior margin of the cepha- 

 lothorax. If the legs still continue their kicking against the in- 

 side of the "vicarious chorion" the embryo is completely 

 turned over and falls back to the lowest position of the sphere, 

 with the haemal side uppermost. 



On the other hand, if the "vicarious chorion" is not held 

 in any way, the same action which in the previous case caused 

 the embryo to turn over within it, causes the egg as a whole to 

 roll slowly over the substrate. That this must be the case is 

 evident if it is remembered that the weight of the embryo tends 

 to maintain the latter in a constant position with reference to 

 the center of the earth, while the hollow sphere, against the in- 

 side of which it is pushing, is rotated about it. On account of 

 the fact that the embryo lies in a fluid in the "vicarious chor- 

 ion" there is practically no internal friction to hinder this rota- 

 tion of the sphere about it. It will be seen that, from the point 

 of view of mechanics, whether the "vicarious chorion" as a 

 whole shall rotate or not, depends on the relation which hap- 

 pens at the time to prevail between the weight of the embryo 

 and the external forces hindering any movement of the sphere. 

 If the weight of the embryo overbalances the external forces 

 the sphere will rotate, while on the other hand, if the external 



^ Of course the terms anterior and posterior are here used to refer to the por- 

 tions of the "vicarious chorion" nearest, at any given time, to the anterior and 

 posterior ends of the embryo. 



