40 THE PINEAL ORGAN 



together, the corresponding parts uniting with each other so as to form a 

 " duplicitas anterior " or a " duplicitas posterior." 



Each of these two hypotheses has assisted in the proper comprehension 

 of the various conditions which are presented in the whole series of 

 anomalies involving duplication of parts, and both are necessary, although 

 in a modified form, in order to appreciate fully the way in which according 

 to the newer conceptions an organizer has the power at an early stage of 

 development to determine from the outset the mode of development of a 

 particular part ; or at a later stage, of overriding the previously deter- 

 mined power of a particular part of the embryonic area to form a particular 

 region or organ, such as that of the eyes, nose, and mouth. The older 

 observations and records of blastoderms showing two separate embryonic 

 axes and the explanations which were put forward to account for these 

 were of great value in solving some of the general problems arising from 

 the occurrence of twin or multiple births and double monsters, and they 

 have in many instances formed the basis on which the more recent work 

 of Spemann and others has been founded. The new conceptions, there- 

 fore, must be regarded as supplementary to rather than as replacing the 

 older conceptions. The latter cleared the way for the newer work by 

 establishing the distinction between uniovular and biovular twins ; the 

 origin of double monsters from a single ovum, in which two gastrular 

 invaginations and embryonic axes were present ; many of the details with 

 regard to gastrular invagination and the formation of the primitive streak 

 and head-process, and other important points, relating to relative rate 

 of growth of particular parts, and also dominance of one part over 

 another. 



Beginning with what is known as the " radiation theory " of Rauber, 

 Fig. 31, A, B, C, represents in a schematic manner three stages in the 

 development of a Duplicitas anterior in the blastoderm of a bony fish. 

 Rauber considered that the occurrence of double monsters was due to the 

 formation of two embryonic buds at the thickened margin of the embryonic 

 disc in place of one. These buds grew towards the centre of the disc in 

 a radial direction. In those cases in which the embryonic axes were 

 exactly opposite to each other the germinal disc would be divided by a 

 vertical plane passing through the longitudinal axis of each embryonic 

 bud, comprising an area including 180 of the whole. If, however, the 

 embryonic buds arose close to each other, as represented in Fig. 31, A, 

 the part of the germinal ridge mn between them, which he termed the 

 " inner intermediate zone," would be much shorter than the rest of the 

 circumference, ystz, which he called the " outer intermediate zone " ; 

 in the formation of the head-regions of the double-monster, the segments 

 mn and yz of the germinal ridge will be added to the original ingrowing 



