ARREST AND EXCESS OF GROWTH. XIX 



The fusion of distinct buds in plants, as in syncarpy 

 (Prep. Nos. 6 to 14), is clearly a condition analogous to 

 the union of embryos originating on a single yelk : in both 

 cases the united growths probably originate by a similar 

 process, the formation of two centres of growth in the place 

 of one. A similar mode of union occurs normally between 

 contiguous zooids in the Gorgonidse, or fan corals. 



In the eggs of birds three distinct conditions occur : 

 First, there may be two yelks enclosed in a single shell ; 

 this condition may give rise to two young, and if they 

 become united they only adhere by heterologous parts, 

 or possibly by the integument of the umbilicus *. Se- 

 condly, there may be two cicatrices on one yelk : in this 

 case the yelk is virtually equivalent to two mammalian 

 yelks, as the cicatrix is the homologue of the mammalian 

 yelk, the great food-yelk being absent in this Class f. Or, 

 thirdly, two embryos may be formed on the same cicatrix : 

 this condition is the most frequent, and gives rise either to 

 twin chicks (?) or to double monsters. 



Dichotomy, as already stated, very rarely affects a limb 

 or digit ; it is liable to occur in the kidneys. 



Doubling of the cavities of the heart, bladder, and uterus 

 are malformations totally unlike those arising from dicho- 

 tomy. The latter organ is certainly doubled by arrest of 

 development. " 



Classes III. & IV. ARREST AND EXCESS OF GROWTH 



Include, first, dwarfs and giants, which are either the effect 

 of variation of nutrition or development ; and, secondly, 

 cases of partial excess or arrest, which are more nearly allied 

 to ordinary disease : these may be considered as cases of 

 nitra-uterine hypertrophy or atrophy. 



* There is no improbability in such union, and it appears to have 

 been observed. M. Camille Dareste thinks this adhesion occurs. 



t E. van Beneden, ' Rccherches sur la composition et la significa- 

 tion de 1'ceuf.' Bruxelles, 1870. 



