66 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



become greater (Lung-fish). In D the secondary continuity of the 

 mesoderm with the endoderm just outside the notochord is present 

 and proliferation of mesoderm cells has commenced in this region 

 (Amphibia). Finally, in E, with the very great increase in the bulk 

 of the yolk, the dorsal part of the embryo is still more flattened out, 

 and the addition to the mesoderm by proliferation of endoderm cells 

 into it close to the notochord has now become conspicuous (Elasmo- 

 branch). 



THE MESENCHYME 



The fate of the mesoderm whose origin has just been traced is to 

 give rise directly to the peritoneal epithelium which lines the body 

 cavity and covers the organs lying within it, and also to the muscular 

 system. Indirectly it, however, also plays a great part in the forma- 

 tion of what is known as the mesenchyme. 



Whereas for a time the Vertebrate body is composed of compact 

 masses or layers of cells, it is a general characteristic that, as develop- 

 ment goes on, individual cells detach themselves and wander away 

 through the body, multiplying by fission accompanied by mitosis, and 

 behaving in fact very much as if they were independent organisms. 

 In the course of the many generations of these cells which arise 

 during the process of individual development, they become divided 

 into various strains which show marked differentiation for the per- 

 formance of different functions. 



Some retain a relatively primitive amoeboid form and undertake 

 such functions as the transport of food material, the absorption of 

 moribund tissues in regions where shrinkage in volume or atrophy is 

 taking place, and the ingestion and destruction of attacking organ- 

 isms such as disease germs. Some, their protoplasm laden with 

 insoluble excretory products as a result of their active metabolism, 

 wander towards the light and settle down near the surface of the 

 body as pigment cells or chromatophores which serve on the one hand 

 to protect the underlying tissues from the light and upon the other 

 to give distinctive coloration to the animal. Others again settle 

 down in an abundant jelly-like intercellular matrix to form connect- 

 ive or packing tissue, which in turn shows evolution in various 

 directions in accordance more particularly with different developments 

 of the intercellular matrix. Of special importance are these types 

 in which the matrix becomes hard and rigid so as to form skeletal 

 tissues such as bone and cartilage. 



Another important strain of these cells is characterized by the 

 fluidity of the matrix and the independence of the individual cells 

 which float in it. This liquid connective tissue forms the blood 

 which, pumped through an elaborate system of vessels, serves on the 

 one hand for the transport of food and oxygen to the tissues, and on 

 the other for carrying away the waste products of metabolism to the 

 special excretory organs the duty of which is finally to remove these 

 harmful substances. 



