340 ROXIE A. WEBER. 



though sometimes two or three, produces an embryo. The 

 remaining ones, not becoming fertilized, disintegrate completely. 

 By a series of somewhat irregular cleavages the egg develops 

 into a germinal bladder, one layer in thickness (PI. IX., Fig. 2, 

 Kleinenberg). Thereupon a two-layered condition begins to 

 appear, one side of the cell mass becoming differentiated in 

 advance of the other. At this pole all the cells divide rapidly 

 except two, which become pushed in and covered over by the 

 small blastomeres. From these two cells (mesoblasts) are 

 derived the first rudiments of entoderm and mesoderm (PI. IX., 



Fig- 4)- 



While this elongation is taking place a transverse furrow 

 appears midway between the tw r o ends extending almost entirely 

 around, leaving the two hemispheres connected by only a few 

 enlarged ectodermal cells. 



When this stage has been reached the cells of the other pole 

 begin to undergo the same changes and finally we have formed 

 from each half an embryo joined in varying degrees to the other 

 by a band of ectodermal cells. The separation is eventually 

 effected by a series of rotations which usually result in the break- 

 ing of the uniting cord. When for one reason or another this 

 is not accomplished we have true monstrosities in all degrees of 

 coalescence. 



According to Kleinenberg the union never extends to the 

 internal organs but is confined to the external epithelium of the 

 body wall. 



The monsters formed in this manner are of the same or nearly 

 the same size. There is however still another type of abnor- 

 mality (PI. IX., Fig. 10), e. g., those showing bud formation. 

 Kleinenberg explains their production as being due to a very 

 unequal development of the two halves of the above mentioned 

 cell mass. 



In all his work he has found only a few cases in which two 

 individuals did not emerge from one capsule and in those few 

 exceptions rudiments of a second were usually found. For this 

 reason he concludes that each egg produces two individuals 

 normally. 



Vedjovsky on the other hand holds this condition to be ab- 



