XV111 INTRODUCTION. 



placenta, or a common amnion and distinct cords, to those 

 which have separate ammo tic sacs but a common chorion. 



It is almost as difficult to conceive that two amniotic 

 sacs from distinct yelks should unite by their edges as it is 

 that two embryos should unite by their visceral laminae 

 when enclosed in distinct amnia. Moreover, the yelk-cap- 

 sule is probably persistent, and a seam at least would be 

 apparent if the amnia were fused at a later period of deve- 

 lopment by the absorption of those portions by which they 

 are in contact. M. Dareste believes that the amniotic sacs 

 produced from distinct yelks become fused together occa- 

 sionally by their edges, and M. Broca has recorded several 

 cases in which two amnia formed from distinct yelks com- 

 municate or form a single amnion *. It would be of extreme 

 importance to discover whether, in such cases, some traces 

 at least of the primitive septum do not remain. 



The formation of double monsters may probably be 

 looked upon, therefore, as an arrested attempt at something 

 very like zooid reproduction, perhaps due to the germinal 

 disk in its earliest state undergoing division, analogous to 

 that which occurs as a normal mode of reproduction amongst 

 the lower forms of life by a kind of reversion to a primitive 

 type. This view will doubtless commend itself to some 

 minds on mature consideration, although it may perhaps 

 appear improbable to others. 



If two complete primitive grooves occur, any arrest in 

 the growth of the blastoderm between them, or their too 

 close approximation at their first formation, would obviously 

 give rise to the subsequent union of the resulting embryos. 

 These views are ably expounded by Dr. B. Schultze, in his 

 essay 'Ueber anomale Duplicitat der Axenorgane'f, where 

 he proposes a subdivision of double malformations on 

 theoretical grounds, which is essentially the same as that 

 adopted in the following pages. 



* C. Dareste, " Sur les ceufs a double germe," Ann. Sc. Nat. 1862, 

 t. xvii. pp. 48 & 49. 



f Virchow's Archiv, Bd. xvii. p. 479. 



