124 B. F. KINGSBURY 



This failure to appreciate the importance of Hertwig's 'blasto- 

 pore theory' for his own 'concrescence theory' seems to lie mainly 

 in his inability to see how the primitive streak could in any way 

 correspond to a blastoporic lip. Minot ('90, '92), on the other 

 hand, who was an energetic champion of the theory of concres- 

 cence, clearly recognized — independently of Hertwig's experi- 

 mental evidence — that ''concrescence is a method of uniting the 

 lips of a greatly elongated gastrula mouth." Since the year 1892 

 investigations upon the early development of the vertebrate 

 body have contributed fact and interpretation both for and 

 against concrescence. The blastopore theory, however, gives us 

 the more fundamental conception. From the dorsal lip of the 

 blastopore, or the primitive streak that clearly represents it 

 essentially, is formed by differential growth in that region, neural 

 plate, notochord, mesoderm. For this the evidence aside from 

 that experimental and teratological, though largely indirect, is 

 cumulative and conclusive. The growth of the blastoporic lip 

 (primitive streak) essentially constitutes a closure of a primitive 

 blastoporic opening by means of a concrescence, actual or poten- 

 tial. The concrescence theory thus loses all force or application 

 aside from the blastopore theory. The sutura neurochordalis 

 only has significance as a line of concrescent closure if it is con- 

 ceived as formed along the line of growth of the dorsal blastoporic 

 lip. 



It seems a little remarkable that His, entertaining as he did 

 the view that the body axis is laid down by concrescence of a 

 germ ring, should have failed to recognize that there was a high 

 probability at least that anterior^ to the line of concrescence the 

 body material should be primarily continuous and that the neu- 

 rochordal suture could not extend fully to the secondary line of 

 closure — the frontal suture closing the neural tube anteriorly. 

 Indeed, in his 1891 paper on concrescence, here and there, par- 

 ticularly in his discussion of the formation of the embryo from 

 the primitive streak (p. 76), as well as in his figures, are indications 

 of his recognition of a 'preaxial' portion of the body, but he never 

 apparently saw the import of this for his own theory. Minot 

 ('92) in this respect clearly appreciated that there is a primitive 



