12 INTRODUCTION 



The mesothelium produces the lining of the coelomic cavities 

 — pericardial, pleural, peritoneal — the reproductive and excretory 

 organs and the voluntary muscles and those of the heart. 



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

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

 of the heart) connective tissue, ligaments, cartilage, bone, and the 

 corpuscles 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 blasto- 

 pore. After it is outlined the plate is rolled into a tube, its front end 

 and lateral margins rising up, forming neural folds («r), between 

 which is the medullary groove {ng). Eventually the folds meet and 

 fuse above, so that a tube results (fig. 6, nc), the cavity of which 

 persists throughout life as the cavities (ventricles) of the brain and 

 the central canal of the spinal cord. From the cells of the walls of 

 the canal the nervous tissue arises. 



This process of infolding progresses from in front backward. For 

 a time, in some vertebrates, a small opening, the anterior neuropore, 

 persists at the anterior end. The infolding extends back to the poste- 

 rior end of the neural plate so that, as will readily be understood, the 

 whole limits of the blastopore are included in the floor of the neural 

 canal. Occasionally the closure of the neural folds is completed 

 before that of the blastopore so that for a short time a short tube, the 

 neurenteric canal (fig. 7), connects the archenteron with the neural 

 canal. Soon after the closure of the neural tube the fused tissue 

 splits horizontally, separating the nervous system from the rest of 

 the ectoderm. Its subsequent history will be traced in the section 

 on the Nervous System. 



