8 INTRODUCTORY EMBRYOLOGY. 



distinguished : the one turned towards the ectoderm is called 

 the somatic or parietal layer ; the one facing the entoderm is 

 the splanchnic or visceral layer. 



Besides this mesoderm arising thus as a continuous out- 

 growth, another type of mesoderm also invades the segmenta- 

 tion cavity. This arises by the migration or inwandering into 

 this space of single cells, which may separate themselves from 

 either entoderm or mesoderm ; or in some instances, as recent 

 investigations tend to show, from the ectoderm as well. Since 

 these two types of mesoderm differ in their origin, and, as will 

 be seen later, in their character and fate, they have been given 

 different names. That mesoderm which bounds the coelomic 

 cavities and all parts formed from it is called mesothelium ; 

 that which arises from the scattered immigrant cells is 

 mesenchyme. 



At this point, where the four germ-layers of the embryo are 

 differentiated from each other, it will be interesting to state 

 what portions of the adult vertebrate are derived from each. 



The ectoderm gives rise to the outer portion (epidermis) of 

 the skin, the outer layer of scales, hair, feathers, the enamel of 

 the teeth, nails, claws, true horn, and the essential parts of all 

 sensory and nervous structures. 



The entoderm develops into the lining of the alimentary 

 canal and the various cavities — gills, lungs, liver, pancreas — • 

 connected with it ; also to the notochord, and possibly to the 

 lining of the blood-vessels. 



From the mesothelium arises the lining of the body cavity, 

 reproductive and excretory organs, and the voluntary muscles 

 (including the muscles of the heart) . 



The mesenchyme produces the deeper layers of the skin, 

 the lower portions of scales, and the dentine of the teeth ; invol- 

 xmtary muscles, connective tissue, fat, cartilage, bone, blood, 

 and lymph corpuscles. 



From the point where the germ-layers are outlined the de- 

 velopment must be traced in two different directions. One line 

 follows out the differentiation of the cells and their grouping 

 into tissues ; the other traces the development of the various 

 organs of the adult. 



