164 THE ANCESTRY OF VERTEBRATES 



call the process, by whtth this is performed, the gastrul- 

 ation, the latter has nothing to do with the invagination 

 following soon afterwards. When the latter sets in, the 

 separation of the two primary germinal layers has been 

 already completed and the opposition between the two pro- 

 cesses is, according to LwOFF's (1894) well-known con- 

 ception, still more accentuated by the fact that not only the 

 entoderm but also a part of the ectoderm would invaginate, 

 forming especially the roof of the archenteron. To this 

 latter view we shall revert in due time. 



A somewhat similar view is held by BRACHET (1902) in 

 his study of the gastrulation in the Amphibian egg. He 

 too distinguishes a "clivage gastruleen", the gastrulation, 

 i. e. the formation of the primary germ layers, being per- 

 formed during cleavage The limit between the entodermal 

 and the ectodeimal area, both still on the surface of the 

 egg, represents a "blastopore virtuel" and, by the appearance 

 of the true blastopore border, the "blastopore reel", gastrul- 

 ation is only completed : "la formation des levres blastopo- 

 rales, aussi bien de la levre dorsale que de la levre ventrale, 

 ne constitue nuUement le debut de la gastrulation, mais en 

 indique plutot, a certains points de vue, I'achevement" (p. 225). 

 It must be added, however, that BRACHET denies an in- 

 vagination of "animal"' cells round the dorsal blastopore 

 border, as postulated by LWOFF, but feels, on the other 

 hand, more inclined to the concrescence-theory. 



HUBRECHT (1890, p. 518) and Keibel (1889, p. 376, 

 1890), mainly from their studies on Amniote embryology, 

 were led to advocate the idea of a gastrulation in two 

 phases, a palingenetic one, to be considered as invagination, 

 and a caenogenetic one, represented by a precocious de- 

 lamination and a splitting of the entoderm-cells. Ontoge- 

 netically the latter occurs first and is followed by the former 

 which in higher Amniotes becomes less and less evident. 

 ASSHETON (1894), a few years later, asserts that in the 

 development of the frog, and in that in Vertebrates in 

 general, two processes must be distinguished: 1. the form- 

 ation of the archenteron by splitting of the endoderm 

 cells, 2. the growing over of the blastopore border, which 

 is no longer to be counted to the gastrulation. Since then 

 HUBRECHT (1902, p. 67, 1905, p. 361), evidently influenced 

 by. ASSHETON and BRACHET, has ceased to recognize the 

 second or palingenelic phase as forming part of the gas- 



