MAMMALIAN DEVELOPMENT 815 



the inner cell mass with it. This is known as implantation. The en- 

 tire blastocyst makes a rapid growth and the cavity becomes distended 

 with the lymphlike fluid. 



Next, a simplified process of gastrulation takes place, in that two 

 successive layers of cells are shed into the blastocyst cavity from the 

 free margin of the inner cell mass. These are layers of endoderm 

 cells. The ends of the layers join to form an elongated enclosure. 

 The inner cells which are joined to the trophoblast become the 

 ectoderm of the body and the trophoblast remains as extraembryonic 

 ectoderm. The cavity enclosed by the endoderm is the archenteron 

 or primitive gut. In coelenterates and sponges development stops 

 with this two-layered or gastrula condition. 



In higher forms, the mesoderm or third germ layer is formed 

 immediately following gastrulation. At this stage in mammals and 

 most other vertebrates a primitive streak appears along the dorsal 

 midline of the posterior portion of the blastoderm (embryo). When 

 sectioned, the primitive streak appears as a thickened band which is 

 continuous with the ectoderm. The ventral side of this streak pro- 

 duces many cells which organize as a sheet of mesoderm at each side 

 and spread both laterally and caudally between the other two germ 

 layers. The mesoderm continues to spread until it finally encircles 

 the endoderm. The mass of cells thus formed, is soon divided into two 

 layers by the development of a small cavity in it at each side of the 

 archenteron. These cavities grow ventrally in the mesoderm until 

 they meet each other at the ventral side of the archenteron. This 

 cavity is the coelom or body cavity. The layer of mesoderm forming 

 the outer or lateral wall (next to the ectoderm) of the coelom is 

 known as somatic or parietal mesoderm and that on the side next to 

 the endoderm is the splanchnic or visceral mesoderm. Later in de- 

 velopment, when the somatic layer unites with the ectoderm, it forms 

 the somatopleure (body wall). Similarly the splanchnic layer unites 

 with the endoderm to form the splanchnopleure or wall of the ali- 

 mentary canal. 



Almost coincidental with the early development of the mesoderm, but 

 independent of it, is the formation of the neural plate as a thicken- 

 ing and depression in the ectoderm along its dorsal midline. This 

 begins just above the primitive streak mentioned previously and 

 grows anteriorly. This is the portion from which the nervous system 

 develops. This plate sinks, neural folds develop along its sides form- 



