[15 
call the process, by which 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 RE, To this 
latter view we shall revert in due tim 
A somewhat similar view is held by BRACHET (1902) in 
his study of the gastrulation in the Amphibian egg. He 
too distinguishes a “clivage gastruléen”, the gastrulation, 
ie. the formation of the primary germ layers, being per- 
formed during cleavage. The limit between the entodermal 
and the ectodermal area, both still on the surface of the 
egg, represents a “blastopore virtuel” and, by the appearance 
of the true blastopore border, the “blastopore réel”, gastrul- 
ation is only completed: “la formation des lêvres blastopo- 
rales, aussi bien de la lèvre dorsale que de la lèvre ventrale, 
ne constitue nullement le début de la gastrulation, mais en 
indique plutôt, a certains points de vue, "achèvement”(p. 225). 
lt 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-celis. 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 palingenetic phase as forming part of the gas- 
