170 THE ANCESTRY OF VERTEBRATES 



of the egg, it follows that the blastopore has also originally 

 nearly the same diameter, and in the figures given by 

 ROUX (1888, p. 698) and Hertwig (1892) for the eggs 

 showing the Spina bifida-phenomenon, this indeed proves 

 to be often the case, the blastopore-border extending round 

 the equator of the egg. In normal eggs, however, the dia- 

 meter of the blastopore, even if we measure the distance 

 between the place where the dorsal rim first appears, and that 

 where the ventral lip appears, never reaches 90° (cf. fig. 35). 

 As we shall see, the pricking experiments made by different 

 authors and by myself lead to conclusions quite different 

 from those of Hertwig and are in no way to be recon- 

 ciled with the latter. Thus, I think Eycleshymer (1895, 

 p. 388) is quite right in questioning the entire evidence 

 adduced from pathological forms and so probably is WILSON 

 (1900) when he suggests that the spina bifida-pheno- 

 menon might have to be interpreted by a kind of rupture 

 or rolling in of the dorsal lip. That something cf the kind 

 occurs is rendered probable by observations on eggs with 

 a tendency to spina bifida which 1 shall mention further 

 on (cf. p. 182). 



As we shall see later, the study of the movement of the 

 blastopore border with the aid of artificial marks proves, 

 that there can be no question about the whole dorsal side 

 of the embryo being formed by concrescence since the 

 greater part lies in front of the place where the dorsal 

 border of the blastopore first appears. In Amphioxus also 

 several authors (LWOFF, 1894, GOETTE, 1895, SOBOTTA, 

 1897, GARBOWSKI, 1898, KLAATSCH, 1898, MACBRIDE, 1898) 

 have emphasized that no evidence in favour of the con- 

 crescence theory can be adduced from the observed facts. 



I shall not deny that concrescence ever plays a role in 

 the closure of the blastopore, especially not in the case 

 of yolk-laden eggs, but this is only a secondary pheno- 

 menon to which no primary phylogenetic significance can 

 be attributed. Also the formation of the primitive streak in 

 Amniotes e. g. must be explained by concrescence of part 

 of the blastopore border which has lost its original 

 character. 



Invagination of ectoderm cells? — LwOFF (1894) has 

 been the principal advocate of another tendency in inter- 

 preting the process of gastrulation in Chordates. As stated 

 above, the entoderm-cells, e. g. in Amphibians, are of unequal 



