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 turned ^ to 

 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 



SC a en <?c 



FIG. 3. Diagram of a typical 

 blastula with central segmentation 

 cavity. 



FIG. 4. -Diagram of a gastrula. 

 a, archenteron; b, blastopore; ec, ecto- 

 derm ;en, en toderm; sc, segmentation 

 cavitv. 



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- 



