EMBRYOLOGY. 9 
Its cells at first show but little differentiation except in size. Next 
follow processes which are to differentiate the cells into layers, charac- 
terized by both position and fate. 
Gastrulation.—In the simplest form this differentiation is brought 
about by an inversion of one-half of the blastula into the other, thus 
more or less completely obliterating the segmentation cavity, much as 
one may push one side of a rubber ball into the other, forming a double- 
walled cup (fig. 4). This stage is called the gastrula, and the process 
of inpushing is invagination. With this the first appearance of the 
structures of the adult is seen. The outer wall of the cup is turnedto 
the external world and thus act as a skin for the embryo. This layer 
is ‘called the ectoderm. The opening or mouth into the cup is the 
° 
Fic. 3.—Diagram of a typical Fic. 4.—Diagram of a gastrula. 
blastula with central segmentation a, archenteron; b, blastopore; ec, ecto- 
cavity. derm; en, entoderm; sc, segmentation 
cavity. 
blastopore. The inside of the cup is well fitted for the digestion of 
food as it can be held together there and the digestive fluids are less 
liable to waste. Hence the cavity is called the archenteron (primitive 
stomach), and the layer of cells which line it is the entoderm. That 
these comparisons are more than analogies of position is shown by 
their fates, the ectoderm forming part of the skin of the adult, the 
entoderm the lining of the digestive tract. Between ectoderm and 
entoderm are the remains of the segmentation cavity, filled with an 
albuminous fluid. It will be convenient later to speak of the line where 
ectoderm and entoderm meet at the blastopore as the ect-ental line. 
Closure of the Blastopore.—Next, the blastopore closes, the 
process beginning at what will be the head end of the embryo and pro- 
