Experiments on Amphioxus Eggs 79 



the J blastomeres will not invaginate. Again, Wilson did find 

 some very small gastrulae "even less than J the normal size," but 

 he says " these gastrulas however did not arise from 8-celled stages 

 but from 2- and 4-celled, and I can only explain their origin by 

 supposing either that the J or -| blastomeres underwent preliminary 

 fission before beginning their progressive development, or that they 

 were mechanically broken into smaller fragments by the operation 

 of shaking as often happens in the case of entire undivided ova." 



Thus a gastrula J or y 1 ^ normal size may be obtained, but 

 these are derived from \ half embryos or J half embryos, in all 

 of which cases the reduction is due to vertical partitions which 

 do not disturb the proportions of upper and lower poles as a 

 horizontal partition does. On the hypothesis which I suggest 

 these observations receive a perfectly comprehensible explanation. 



Granted that there is such a form of energy we must inquire 

 whether the attractive property is constant or whether it is only 

 temporary or whether it is varying in intensity. For instance, it 

 might be more intense after the complete formation of each cell 

 or blastomere, and gradually diminish with the age of the cell, 

 while passing gradually into the bipolar state to become again 

 more evident as an attractive force after the recurrence of cell 

 division. 



Anyone who is familiar with the fresh segmenting ova of 

 mammals will remember how when first found presumably 

 while still alive the segments appear pressed together so that 

 the sides in contact with other cells are flattened, but that after 

 a time, when presumably the cells have died, this flattening 

 against one another disappears, each segment at any rate in the 

 2- and 4-celled stages becomes spherical instead of oval and 

 touches its neighbouring segments at points only. 



Fig. 38 shows the 2-segment stage of a rabbit. Each segment 

 is flattened against the other, but this is not due to insufficiency of 

 space within the zona radiata because the cells do not touch the 

 zona on all sides and I do not see how it can be explained by 

 reference to capillarity or surface tension or cements, because such 

 causes ought to have the same effect after death as before. It 

 may be objected that the change from the oval closely pressed 

 together condition is due to a lower temperature bringing about a 



