XVI INTRODUCTION. 



a revenir au type primitif de 1'espece" *. Lereboullet states 

 that this condition of the germinal disk gives rise to the 

 formation of a more or less completely double primitive 

 groove and notochord, although the tendency to revert to 

 the primitive type often causes the two grooves to coalesce at 

 one or both extremities, or throughout their entire length. 



II. The second Subclass, Homologous union, consists of 

 monsters formed by the union of two distinct embryos 

 adherent by the fusion of homologous parts ; each rib of 

 the one, for instance, becomes fused with the corresponding 

 rib of the other, and the other organs are all similarly 

 united. The fact that such embryos originate in all cases 

 upon a single yelk has been strongly insisted upon by most 

 teratologists since the time of Vrolik, and double embryos 

 on one yelk have been observed in numerous instances. 

 Whenever the structures formed from the inner layer of 

 the blastoderm, the intestinal canal and liver, are continuous, 

 as they are in every case in the collection as well as in 

 nearly if not in all the cases on record, it is impossible to 

 conceive that the embryos could have originated in any 

 other manner, since the adhesion of the parietes of the 

 embryos by their external surfaces could not account for 

 the continuity of the alimentary canal. M. Canaille 

 Dareste believes, however, that cases of homologous union 

 occur between embryos formed on separate yelks, oc- 

 casionally at least. 



In homologous union the dorsal groove, instead of 

 being only partially double, as in dichotomy, is completely 

 double, and the two embryos are formed so close to each 

 other that the visceral laminae become united. As union 

 takes place before the differentiation of distinct organs oc- 

 curs, these are formed in continuity with each other. The 

 differentiation is probably gradual ; the ribs, for instance, 

 instead of growing forwards from the spine and meeting 

 in the mesial line, are probably gradually evolved from the 

 cells of the visceral laminae. This evolution is continuous 

 * Comptes Rendus, 1855, pp. 885 and 1029. 



