lUl! Mil. NOEO TAYLKU OX A UNIQUE CASE OF 



portions of the blastoderm. At the early stage at which the 

 disturbance must have occurred, there can hardly have been 

 any sharp distinction between the embryonal and extra- 

 embryonal portions of the blastoderm : hence the presence 

 of a second centre of embryonal formation clearly implies the 

 existence of a region of blastodermic extension and growth 

 in correlation with it, quite distinct from that related to the 

 main embryonal rudiment (Rudiment a). The existence of 

 such a centre also explains the great length of the blasto- 

 derm as a Avhole, composed as it is of two adjacent areas of 

 l)lastodermic extension lying along one longitudinal axis, and 

 furthei" explains the great separation of the two rudiments, 

 since the rate of extension of that portion of the blasto- 

 derm lying between tliem must of necessity be considerably 

 greater than in i\.ny other area, for here there is increase of 

 blastodermic extent in relation to each of the embryonal 

 formations. 



IV. Teratological Significance. 



1. Searing on the nature of imequal monstrosity, 

 " Duplicitas asymmetros." 



Unequal double monsters are those in which the two com- 

 ponents show marked dissimilarity in the degree of their 

 development. They are best known in mammals, particularly in 

 man, in which numerous types have been recorded, Tliere 

 appears, however, to be little knowledge as to the earlier stages 

 of these forms of double monstrosity, hence little would seem to 

 be definitely established as to their mode of development, though 

 there has been considerable speculation on these questions. Very 

 few cases of unequal double monsti'osity in bii'ds seem to have 

 been put upon record, and hei'e, too, little is known as to their 

 mode of oi'igin and development. Kaestner, one of the foremost 

 students of double monstrosity in birds, remarks (6, 7) that, 

 " Ueber die embryonale Entwicklung parasitiiren oder, wie man 

 sie auch nennt, asymmeti'ischen Doppelbildungen wissen wir 

 wenig " ; while Wilder (18), speaking of mammals, said : " To 

 my knowledge this form of monster has never been studied for 

 the purpose of testing whether or not the two components were 

 ever originally physical duplicates." 



The accurate definition of unequal double monstrosity is a 

 matter of some difficulty in the absence of knowledge as to their 

 mode of origin and development. 



CJeoffro}' 8t. Hilaire, who jdaced them in the second order of 

 his group of " Monstres Doubles," characterised them as 

 follows (15) ;— 



" Monstres doubles parasitaires ou composes de deux individus 

 tres-inegaux et tres-dissemblables I'un coraplet ou presque com- 

 plet, analogue a un autosite ; I'autre non seulement beaucoup 

 plus petit, mais ties-imparfait, analogue a uu omphalosite ou 



