lO INTRODUCTION 



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 appear- 

 ance of the structures of the adult is seen. The outer wall of the cup 

 is turned to the external world and thus acts as a skin for the embryo. 

 This layer is called the ectoderm. The opening or mouth into the 

 cup is the 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 archen- 

 teron (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. Be- 

 tween 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 blasto- 

 pore as the ect-ental line. 



Closure of the Blastopore. — Next, the blastopore closes, the proc- 

 ess beginning at what will be the head end of the embryo and pro- 

 ceeding gradually backward. Usually the closure is complete, but 

 occasionally the hinder part remains open and forms the anus. 

 Where it closes completely the vent is subsequently formed in the 

 line of closure. This union of the two lips of the blastopore in clos- 

 ing marks the middle line of the back of the future animal, and is 

 called at first the primitive groove, the region on either side of it being 

 known as the primitive streak, terms of importance in understanding 

 the gastrulation of the higher vertebrates. 



Mesoderm. — With the closure of the blastopore the embryo elon- 

 gates and the archenteron is converted into a tube. Next, from the 

 region of closure and from the entodermal layer, a fold of cells grows 

 in on either side between ectoderm and entoderm, thus farther en- 

 croaching on the segmentation cavity. These cells form the middle 

 layer or mesoderm. Inside this fold is a space, connected at first 

 with the archenteron (fig. 5), but soon the cavity of each side is cut 

 off by a growing together of the opening into the archenteron and is 



