326 
E. W. MACBRIDE. 
of the lumen into two parts is of no more importance than 
the solidity of the oesophagus in many Vertebrate embryos. 
The development of the lower Amniota has been worked 
out in several fortns in great detail. The work of Will on 
the European Gecko Platydactylus (36), of Mitsukuri on 
the turtle Chelone (29), and by Ballowitz on the snake 
Tropidonotus (4) have given very concordant results. In 
all three cases a dorsal lip of the blastopore is formed, not at 
the extreme edge of tlie germ-disc, but inside it, and we are 
apparently justified in assuming that the early growth of the 
ectoderm from the opposite and anterior side of the germ- 
disc, which corresponds to the point y in the gastrula of 
Amphioxus, and its short-circuiting round a small circle 
of the sphere represented by the whole egg, has taken place 
far earlier than it does in the case of the Elasmobranch. 
If this be granted, the subsequent development bears a strong 
resemblance to that of Hypogeophis. In all three cases a 
sac-like cavity forms by invagination beneath the blastoporal 
lip {a, text-fig. 8). Another cavity forms by the separation 
from the yolk of the endoderm cells, which have differentiated 
themselves from the lowest layer in the germinal disc {s'., 
text-fig. 8). This space, coiTesponding to the so-called seg- 
mentation cavity of Hypogeophis, breaks through into the 
invaginated sac ; so an archenteron is formed, the roof of 
which is formed of the invaginated cells, the floor of yolk, and 
the sides and anterior slope of “ secondary endoderm.” After 
the floor of the invaginated sac and the roof of the secondary 
endoderm cavity have met and undergone absorption, three 
parallel folds are formed in the roof of the compound cavity. 
The centre one gives rise to the notochord, the two lateral to 
the trunk coelom on each side. In this way the roof of the 
archenteron is used up, and the sides of the compound 
archenteron unite beneath it in the middle line to constitute 
the roof of the definitive gut. In the higher Amniota, such 
as Aves, the invaginated sac is replaced by an almost or 
quite solid in-growth of cells, and the dorsal lip of the blasto- 
pore becomes drawn out into a slit-like form, the well-known 
