46 THE PINEAL ORGAN 



two axes meet the primary dorso-ventral planes form an angle with each 

 other, the resulting secondary surfaces will be unequal, as in Fig. 23, A 

 and B, and there will be a tendency towards the suppression of the central 

 parts on one side, with the production of such deformities as cyclops and 

 synotia. Asymmetry or inequality may also arise from a difference in 

 the size or vitality of the two growth-centres. Finally, as is suggested 

 by experimental interference with the development of normal ova, the 

 blastomeres or the organizing centre may be divided into two or more 

 growth-centres by constriction or other means and the developmental 

 rudiment of a double organism initiated. Moreover, as previously con- 

 jectured, it seems likely that alterations in the physico-chemical environ- 

 ment of the ovum in the gastrular or pre-gastrular stages due to injury 

 or disease may produce similar defects under natural or non-experimental 

 conditions in both non-placental and placental animals. 



The next question which it is necessary to consider is, Can the causes 

 of these general defects be limited in their action to the development of 

 particular organs ? And more particularly with reference to our present 

 thesis ; Can primarily paired organs such as the nose, eyes, or ears lose 

 their function, atrophy, and either become fused into a single organ or 

 disappear altogether ? These questions involve another, namely, the 

 inheritance of general and localized defects, such as albinism and haemo- 

 philia on the one hand and the inability to complete the development of 

 particular organs on the other hand. As examples of the latter, we may 

 mention the rudimentary teeth which have been found in the jaws of 

 foetal whales and in the curious duck-billed Platypus (Ornithorhynchus) ; 

 or, again, the vestigial limbs found in certain snakes, the vestigial wing- 

 bones of the kiwi, the New Zealand Apteryx ; or Stiedas organ in Rana 

 temporaria (Figs. 162, 163, and 167, Chap. 19, pp. 228, 229, 233), and 

 the median eyes of many invertebrates. 



Associated with these questions is also that of change of function. Of 

 this we have fewer examples than is generally supposed, for although a 

 forelimb may be modified for use as a paddle, a leg, or a wing, its use as 

 an organ of locomotion has not been altered ; perhaps one of the best- 

 known instances of change of function is the conversion of the swim- 

 bladder of fishes, a hydrostatic organ, into the air-breathing lungs of 

 terrestrial animals. Now it sometimes happens that it is easier to build 

 an entirely new structure than to improve and adapt an old one ; the latter 

 is either destroyed in order to make room for the new building or is allowed 

 to fall into decay. The latter alternative seems to have happened in the 

 replacement of one organ by another in the course of evolution which 

 sometimes happens in the animal kingdom, e.g. in the replacement of the 

 median eyes of invertebrates by the lateral eyes, but as we shall see later, 



