VII.] THE MESOBLAST. 199 



ently are in striking contrast with the nerve-cells in the brain 

 and cord. The fibres constituting the white matter of both 

 brain and spinal cord are also probably derived from mesoblast. 



The generative and urinary organs are entirely derived 

 from the mesoblast. It is worthy of notice that their epithe- 

 lium, though resembling so closely the hypoblastic epithelium 

 of the alimentary canal, is distinctly mesoblastic. 



From the mesoblast lastly are derived all the muscular, 

 connective, and nervous and vascular elements, as well of the 

 alimentary canal and its appendages as of the skin and the 

 tegumentary organs. Just as it is only the epidermic moiety of 

 the latter which is derived from the epiblast, so it is only the 

 epithelium of the former which comes fi-om the hypoblast. 



In the present state of our knowledge we cannot in all 

 cases with certainty say which parts of the mesoblast enter 

 into the formation of particular organs ; the more important 

 facts in this part of our subject will however already have 

 been gathered, from the earlier part of this work. 



11. The important events then which characterize the 

 fifth day are : — 



1. The growth of the allantois. 



2. The appearance of the knee and elbow, and of the 

 cartilages which precede the bones of the digits and limbs. 



3. The formation of the primitive cartilaginous cranium, 

 more especially of the investing mass, the trabeculse, and the 

 ethmo-vomeriae plate ; and the appearance of rods of cartilage 

 in the visceral arches. 



4. The developments of the parts of the face : the 

 closing in of the nasal passages by the nasal processes. 



5. The formation of the anus. 



6. A large development of grey matter in the spinal 

 cord as the anterior and posterior cornua ; considerable growth 

 both of the anterior and posterior white columns, and the 

 commencement of the anterior and posterior fissures. 



7. The appearance of the auricular septum, of a septum 

 in the bulbus arteriosus, and of the semilunar valves. 



8. The establishment of the several tissues. 



