384 c. 0. wiriTaMAN. 



"If, therefore, llis's and llauber's view is accepted, it will 

 have to be maintained that only a small part of the body is 

 formed by concrescence, while the larger posterior part grows 

 by intussusception." 



3. The blastopore in Amphioxus is not coextensive with 

 the neural groove, for it is nearly closed before the groove 

 appears. 



4. According to His and Rauber, " the whole of the dorsal, 

 as well as of the ventral wall of the embryo, must be formed 

 by the concrescence of the lips of the blastopore, which is clearly 

 a reductio ad absurd um of the whole theory." 



5. According to Kupffer (No. 31) the epibolic growth of the 

 blastoderm in Clupea and Gasterosteus is equally rapid on all 

 sides until the equatorial line of the egg is passed ; and Balfour 

 considers this to be ''absolutely inconsistent with the con- 

 crescence theory." 



These five arguments probably include all there is of 

 importance to be said against concrescence. Although some 

 of them may claim to be based on comparative embryology, 

 not one of them, nor all of them together, are broad enough to 

 cover the ground embraced in Balfour's theory of the origin of 

 vertebrates. No attempt is made to discuss the two opposed 

 theories on the basis of the now generally received view, that 

 the ancestral form of the vertebrates was an Annelid; and it 

 seems to me surprising that an advocate of this view should 

 leave the Annelids almost entirely out of consideration. Bal- 

 four makes the Elasmobranch embryo the point of departure, 

 and evidently because the primitive type of development has 

 probably suffered less modification here than in the amniotic 

 vertebrate. On similar grounds we may ask, Why not make 

 the Annelid type of development our starting-point, since the 

 mode of development may be presumed to have been conserved 

 in its greatest purity in those animals that have made the 

 smallest departures from the ancestral form ? If there is any 

 truth in the supposed genealogical relationship between verte- 

 brates and Annelids we have certainly a right to expect some 

 fundamental agreement in their modes of development. 



