DICHOTOMY. 115 



monsters, and endeavour to show that they arise from 

 dichotomy, partial or complete, of the trunk-axis of the 

 embryo. Before describing actual specimens it will be 

 well to adduce facts in support of the view that double 

 embryos may arise from a single ovum. 



In 1869 Haeckel * showed that it was possible, by 



FIG. 63. A Sheep with supernumerary fore-leg. 



dividing artificially the eggs of Crystallodes rigidum to 

 multiply the number of embryos. The experiments 

 were very simple, and consisted in taking a Crystal- 

 lodes larva on the second day, in which the amoeboid 

 movements were very lively, placing them in a watch- 

 glass with sea-water, and with the aid of a simple micro- 



1869. 



Zur Entwicklungsgeschichte des Siphonophoren." Utrecht, 



