6 COMPARATIVE ANATOMY 



a central space, the primitive digestive cavity (arckenterpn). The 

 opening of the latter to the exterior, where the two germinal 

 layers are continuous, represents the primitive mouth or Nastopore, 

 which is represented to a certain extent by the primitive streak of 

 higher forms. 



From the ectoderm arise the epiderm and its derivatives, the 

 entire nervous system, the sensory cells, the lens and certain muscles 

 of the eye, the oral and anal involutions (stomodceum and procto- 

 dceuiii), and the oral portion of the pituitary body attached to the 

 brain. In an early stage the endoderm gives rise to an axial rod, 

 the notochord (Figs. 6 and 7), and eventually to the epithelium of the 



Z'At- 



I o 



FIG. 5. GASTBULA. 

 Blp, blastopore ; Ekt, ectoderm ; Ent, endoderm ; U, archenteron. 



greater part of the alimentary canal (Figs. 6 and 10) and its 

 glands, including the thyroid, thymus, liver and pancreas, as well 

 as to the epithelial parts of the gill-sacs and lungs. 



Though we may look upon the ectoderm and endoderm that 

 is, both the primary germinal layers as arising primitively in the 

 manner above described, various modifications occur, depending 

 largely on the type of segmentation, and known as overgrowth 

 (epiboly\ dclamination, and partial delamination. The middle layer 

 or mesoderm is a secondary formation, and is phylogenetically 

 younger than the other two germinal layers ; both as regards the 

 origin of its cells and histologically, it is of a compound nature, 

 and thus forms a marked contrast to the two germinal layers proper. 

 One of its first and most important functions is the formation of 

 blood-cells ; later it gives rise to the heart, vessels, and to nearly 

 all the supporting and connecting substances (connective tissue, 

 adipose tissue, cartilage, bone), serous membranes (peritoneum, 



