TYPES OF VERTEBRATE AND INVERTEBRATE EYES 41 



embryonic buds as represented in Fig. 31, B ; whereas when the whole 

 of the inner intermediate zone mn had been utilized, the remaining 

 segments of the ridge st would come in contact and give rise to the single 



k z k 1 



D 



k 2 k' 



F 



Fig. 31. 



A 3 B, C — Scheme indicating the mode of formation of Duplicitas anterior according 

 to Rauber's theory. ab : vertical or antero-posterior diameter dividing 

 the germinal disc or blastoderm into right and left halves and corresponding 

 to the principal, or dorsoventral, longitudinal axis of the body of the future 

 larva ; hh : head-ends of the embryonic " outgrowths " ; mn : the right 

 and left halves of the inner intermediate segment. These, with the corres- 

 ponding parts of the embryonic outgrowths, will give rise to the internal or 

 opposed halves of the two head-regions of the double monster ; yz : the 

 corresponding outer segments of the margin of the germinal disc ; these 

 will form the external (left and right) halves of the twin head and neck 

 regions ; st : the remaining parts of the germinal ring, which by uniting 

 directly with each other will form the right and left halves of the single body 

 and tail of the abnormal embryo. 



D, E, F — Figures illustrating the formation of Duplicitas anterior in the salmon's 

 egg from two gastrular invaginations, after Hertwig, in accordance with the 

 " concrescence theory " of His. This conception differs from Rauber's in 

 that Hertwig considered that the multiple formations are due to multiple 

 gastrular invaginations at the margin of a single germinal disc, whereas 

 Rauber considered the whole germinal ring as being the boundary of the 

 gastrula-mouth or blastopore. Z : intermediate segment between the two 

 gastrular invaginations ; k\ k- : right and left head rudiments of a double 

 monster; itzv 1 , -, 3 : interrupted lines indicating the expansion of the 

 germinal disc. 



body and tail, by fusing with each other in the median dorso-ventral 

 plane, as appears in C, in which a complete Duplicitas anterior is 

 represented. In this and the preceding drawing it will be seen that when 



