630 WILSON. 



PLATE XXXIV. 



[All from preparations ; 270 diameters.] A series of gastrulas, derived from shaken 2-celled stages, 



7 hours. All from the same spawning. 



Fig. 63. Normal gastruia. 



Figs. 64 to 68. Series leading from slightly expanded form to true double 

 gastrulas. 



Fig. 69. Two separate twins, within one membrane. 



Figs. 70 to 74. Show various relations of the axes and blastopores. 



Fig. 75. In this case the two archentera are not separated by a layer of ecto- 

 blast {cf. Fig. 72). 



Fig. 76. One of the bodies fits like a cap upon one side of the other. 



