542 EMBRYOLOGY. 



currents of the fluid, the blood-cells and lymph-cells, demand 

 renewal the more frequently the more complex the metastasis 

 becomes. This leads to the formation of special breeding places, as 

 it were, for the lymph-corpuscles. In the course of the lymphatic 

 vessels and spaces there takes place at certain points in the con- 

 nective tissue an especially active proliferation of cells. The 

 substance of the connective-tissue framework assumes here the 

 special modification of reticular or adenoid tissue. The surplus of 

 cells produced enters into the lymphatic current as it sweeps past. 

 According as these lymphoid organs exhibit a simple or a complicated 

 structure, they are distinguished as solitary or aggregated follicles, as 

 lymphatic ganglia and spleen. 



Finally there are formed at very many places in the intermediate 

 layer, as especially in the whole course of the intestinal canal, 

 organic [non-striate] muscles. 



After this brief survey of the processes of differentiation in the 

 intermediate layer, which are primarily of an histological nature, I 

 turn to the special history of the development of the organs which 

 arise from it, the blood-vessel and skeletal systems. 



I. The Development of the Blood-vessel System. 



The very first fundament of the blood-vessels and the blood has 

 already been treated of in the first part of this text-book. We will 

 therefore here concern ourselves with the special conditions of the 

 vascular system, with the origin of the heart and chief blood-vessels, 

 and with the special forms which the circulation presents in the 

 various stages of development, and which are dependent on the 

 formation of the foetal membranes. In this I shall treat separately, 

 both for the heart and for the rest of the vascular system, the first 

 fundamental processes of development and the succeeding altera- 

 tions, from which the ultimate condition is finally evolved. 



A. The first DeveloyMientcd Conditions of the Vascular System. 



(a) Of the Heart. 



The vascular system of Vertebrates can be referred back to a very 

 simple fundamental form namely, to two blood-vessel trunks of 

 which the one runs above and the other below the intestine in the 

 direction of the longitudinal axis of the body. The dorsal trunk, the 



