EMBRYOLOGY. I I 



chyme which also comes to lie in the segmentation cavity. This 

 mesenchyme arises as separate cells, coming largely from the mesothe- 

 lium, and to a less extent from the entoderm (see p. 16). Whether 

 any arises from the ectoderm is disputed. 



The Germ Layers. Ectoderm, entoderm and the two types of 

 mesoderm are called the germ layers, because in the animals first 

 studied they were arranged like layers one on the other. Each plays 

 its part in the formation of the adult and gives rise to its peculiar 

 structures. 



The ectoderm forms the outer layer of the skin, hair, claws, feathers, 

 the outer layer of scales, enamel of teeth, and the essential or character- 

 istic part of all sensory and nervous structures. 



The entoderm gives rise to the lining of the digestive tract, and the 

 various outgrowths gills, lungs, liver, pancreas, etc. connected with 

 it. The notochord is also entodermal and possibly the lining of the 

 blood-vessels is derived from this layer. 



The mesothelium produces the lining of the ccelomic cavities 

 pericardial, pleural, peritoneal the reproductive and excretory 

 organs and the voluntary muscles and those of the heart. 



The mesenchyme develops the deeper layer (corium) of the skin 

 and of scales, the dentine of teeth, involuntary muscles (except those of 

 the heart) connective tissue, ligaments, cartilage, bone, and the corpus- 

 cles of blood and lymph. 



In the development of the embryo several processes of differen- 

 tiation occur simultaneously, but in the written account one has to 

 follow another. Hence it must be understood that the modifications 

 described here may be taking place at the same time. 



The Central Nervous System. During the closure of the blasto- 

 pore the ectoderm in front and to either side of the blastoporal lips 

 becomes thickened, the cells elongating at right angles to the surface 

 and becoming cylindrical or fusiform. These cells form the neural or 

 medullary plate (fig. 5, np), sharply marked off from the surrounding 

 cells, which are more flattened, and which eventually are concerned in 

 the formation of the outer layer (epidermis) of the skin. The neural 

 plate is to develop into the brain and the spinal cord, and it is to be 

 noted that later it extends around the hinder end of the blastopore. 

 After it is outlined the plate is rolled into a tube, its front end and lateral 

 margins rising up, forming neural folds (nr], between which is the 

 medullary groove. Eventually the folds meet and fuse above so that 



