TYPES OF VERTEBRATE AND INVERTEBRATE EYES 45 



This statement is, however, challenged by Schwalbe, who gives as an 

 example to the contrary cases of slight Duplicitas anterior (Diprosopus) 

 in which some parts are not incompletely doubled. It may be noted, 

 however, that in the case of Duplicitas anterior (Diprosopus) shown in 

 Fig. 25, although the body on superficial inspection appears to be single, 

 an X-ray photograph showed two vertebral columns extending as far as 

 the pelvis. According to Kaestner's view, this case would represent 

 incomplete doubling. 



Kaestner states that if two primitive streaks are present, the axes of 

 two medullary grooves will also be laid down and the position of two fore- 

 gut grooves determined. If the two primitive grooves are not sufficiently 

 separated from each other to allow of all the organs of the two embryonic 

 rudiments to be infolded, an arrest of development occurs which may be 

 compared with the interference of two wave-systems and the changed 

 conditions of wave-form occasioned by this. Thus if two primitive 

 streaks are sufficiently far apart, two complete medullary grooves will be 

 formed, with a notochord situated below the floor of each groove. If 

 the primitive streaks are very close together there will be one double 

 medullary groove which is wider than normal, and shows two subsidiary 

 grooves lying parallel with each other in its floor. The subsidiary grooves 

 indicate its composite double character, and beneath each of these is a 

 notochord. When the medullary folds which flank the main double 

 medullary groove unite, a single neural tube will be enclosed. This is, 

 however, not a simple tube, but is compound in nature. At the extreme 

 anterior end a double bud is present which has the power to form two 

 separate forebrains. If the embryonic axes are still more closely approxi- 

 mated, a single forebrain will be formed, although developed from two 

 embryonic axes. Kaestner states that many authors would describe the 

 latter condition as due to the fusion of two medullary tubes, but " no — 

 they are, on the contrary, incompletely separated." 



Rabaud puts a slightly different interpretation on this process, namely 

 that in dual or twin developments we have a differentiation within a 

 common region of two developmental centres. 



This view is very similar to the modern conception of two organizers 

 dominating a region which has not yet been completely determined, and 

 which would under normal circumstances give rise to a different structure. 

 Where the spread of the two centres is interfered with by mutual contact, 

 as in Fig. 26, B, further growth in that direction is arrested or proceeds 

 in a different direction. If the primary dorso- ventral planes of the two 

 embryonic axes are in the same plane, the resulting double-monster will 

 be symmetrically developed ; but if the primary dorso-ventral planes 

 of the two embryonic axes are disposed in different planes, so that when the 



