98 MR. NUEL XAYLER ON A UNIQUE CASE OF 



liave constituted the anterior floor of the gut, beneath the fore- 

 brain as a prechordal prolongation of the gut, resulted from the 

 fact that gut-formation was effected by the formation and expan- 

 sion of the pleuro-pericardial coelom alone and not by head-fold 

 formation. This is evinced by the anterior extension of the 

 median pleuro-pericardial cavity (fig. 7 B, Ant.p.j).G.). 



3. The I'etention of this original condition was no doubt a 

 result of the absence of the normal expansion and flexure of the 

 fore-brain. This latter, in the normal embryo, it must be recol- 

 lected, conditions, largely, the depression of the oral plate beneath 

 the ventral surface of the head; and had the growth of the 

 fore-brain occurred in the usual manner in this case, no doubt it 

 would have tended (though it seems doubtful whether it would 

 have succeeded in eflfecting this) to drag round at least a por- 

 tion of the apparently dorsal wall of the pre-infundibular gut 

 (fig. 7 B, Ant.ext.F.G.) into a ventral position. 



Part B, 1'he Origin and Helationship of the Two Embryonal 

 Rudiments. 



■ There seem good grounds, then, for the conclusion that the 

 structure of embryonal Rudiment a is only explicable as resulting 

 from a very early germinal defect, affecting in particular, it 

 Avould seem, the anterior medullary plate region. Turning now 

 to the anterior rudiment, designated in the descriptive part as 

 Kudiment /3, this can only be described as primitive streak-like, 

 so far does it deviate from the normal. There would seem here 

 to be no question but that it must have taken origin from a veiy 

 defective germinal centre compared with that from which a 

 normal embryo is derived. 



Blastoderm E, then, must have been characterised from a 

 veiy early stage by the presence of two germinal centres, both 

 defective presumably, in the sense that they must have lacked 

 some portion of the germinal or formative material present in 

 the normal germ. 



It is proposed to consider here, as briefly as possible, the 

 probable mode of origin of this digerminal blastoderm. 



In the first place, it may be emphasised that while all di- 

 embryonal blastoderms must have been characterised at an earlier 

 stage by the presence of two germinal centres, Specimen E 

 difi'ers from the great majority of those hitherto described in the 

 important respect that each of these germinal centres must have 

 been defeciive, and in this section we shall be concerned almost 

 solely with demonstrating that this defectiveness results from the 

 mode of origin of the two centres, and is only explicable on the 

 supposition of a certain mode of origin. 



Twinning or duplicity is of two main types, to which the 

 terms Dizygotic and Monozygotic have been applied, practically 

 all the multitudinous theories which have been suggested falling 

 into one or other of these two categories. The former sic'nifies 



