INTRODUCTION. 7 



involution of the thickened dorsal region of the embryo (medul- 

 lary plate), which soon becomes constricted off from the epiblast 

 in the form of a hollow tube the medullary cord or tube 

 (comp. Figs. 6 and 7), from which the brain and spinal cord are 

 formed. The hypoblast gives rise eventually to the epithelium of 

 the alimentary canal (Fig. 7, A and B, Ent) with its glands, as well 

 as to the epithelial parts of the lungs, thyroid and thymus glands, 

 liver, and pancreas. 



Though we can look upon the epiblast and hypoblast, that is, both the 

 primary germinal layers, 1 as arising in the manner above described, the 

 problem as to the origin of the mesoblast is as yet by no means settled. All 

 that can be said at present is briefly as follows : The mesoblast is a secondary 



FIG. 6. EARLY STAGE IN THE DIFFERENTIATION OF THE EMBRYO. 



BD, blastoderm ; KS, germinal disk ; KA, body-walls ; B, medullary cord, right 

 and left of which are seen the mesoblastic somites ; G, brain. 



formation, and phylogenetically younger than the other two germinal layers. 

 Reminding one in many points of the "mesenchyma" of Invertebrates, it 

 always arises at first from the point where epiblast and hypoblast pass into one 

 another, that is, from the region of the blastopore, or, what comes to the same 

 thing in the higher Vertebrates, from the primitive streak. Originating 

 between the other two layers, its first and most important function is the 

 formation of blood-corpuscles first of white cells (leucocytes, lymph-cor- 

 puscles); later it gives rise to the heart, blood-vessels, supporting and 

 connective substances (connective- tissue, adipose tissue, cartilage, and bone), 

 serous membranes (peritoneum, pleura, pericardium, arachnoid), excretory and 

 reproductive apparatus, and muscles. 



1 It must be observed that this important difference in the histological differentia- 

 tion of the individual germinal layers cannot be so definitely stated as regards the 

 whole animal kingdom : in certain types of Invertebrates it is not so strongly 

 marked. 



