TYPES OF VERTEBRATE AND INVERTEBRATE EYES 43 



of the cellular material of the germinal ring is utilized in the formation 

 of the head-region, and that the greater part of the ring contains the 

 material for the lateral and ventral parts of the body of the embryo. 

 During the expansion of the germinal disc there occurs in the first place 



Fig. 32. — Schematic Drawings illustrating the Formation of a Double- 

 headed Monster, Duplicitas anterior, on the Basis of the Concrescence 

 Theory. (After Kopsch.) 



A — Germinal disc with two embryonic rudiments at the " lozenge-shaped 

 stage of development and situated close together. The inner intermediate 

 segment is indicated by Roman figures ; the outer segment by Arabic figures. 

 k' 1 , k 1 : head-ends of the embryonic rudiments. 



In B the whole of the intermediate segment has been utilized in the formation of 

 the opposed sides of the twin head-ends of the Duplicitas anterior. 



In C the " inner " intermediate zone, no longer being available for the formation 

 of the embryo, the outer segments of the growing margin of the germinal disc 

 meet and fuse to form the single body and tail-end, as in normal development. 



If the two embryonic rudiments were situated directly opposite each other, it is 

 assumed that two completely separate uniovular twins would be formed — 

 the so-called " inner " and " outer " intermediate segments being equal in 

 extent and sufficient to form two whole embrvos. 



a drawing together of the head-forming regions from which, by union 

 in the median axis of the embryo, the right and left halves of the head are 

 produced. The projecting embryonic bud contains the canalis neuren- 

 tericus in the region of Hensen's node, and later there is formed, by a 



