INTRODUCTION. 



5. Variations in the disposition of the blood-vessels, and 

 especially of the large trunks given off from the aorta. 



Class II. DUPLICITY. 



This Class contains a vast number of very remarkable 

 conditions, at first sight extremely dissimilar, but clearly, on 

 closer study, intimately related in their mode of origin. 

 The forms included in this Class have been divided into 

 three Subclasses : 



Subclass I. Dichotomy. 



II. Homologous union. 

 III. Heterologous union. 



I. Dichotomy, or fission as it is frequently called, con- 

 sists in the formation of two growing points in the place 

 of one, in such a manner as to produce a dichotomously 

 divided embryo or part, each half of the doubled portion 

 being complete in itself. This process occurs normally in 

 the production of a branching axis in plants, and is not un- 

 frequent in the multiplication of Ccelenterates, in the 

 division of Coral-polypes and certain branched Hydrozoa. 

 The use of the term fission to designate this condition is 

 unfortunate, as it implies the division of an already formed 

 part ; whilst Dichotomy in both animals and plants, either 

 normal or abnormal, as has been carefully pointed out by 

 Dr. Maxwell T. Masters in his 'Vegetable Teratology/ 

 consists in the simultaneous production of two points 

 of growth. Fission amongst coral-polypes is of this kind, 

 a fact strongly insisted on by Milne-Edwards. The term 

 has, indeed, been used to designate a totally distinct 

 phenomenon in the Infusoria, where an already formed 

 organism undergoes division. Dichotomy may probably 

 arise from a cleavage of the original cells, out of which a 

 part is developed ; but we know nothing certainly on this 

 point. 



