294 



HISTOGENESIS 



produced by the union of ectoplasmic granules, but this view is not supported by 



either Mall or Spalteholz. 



Adipose Tissue. Certain of the mesenchymal cells give rise not to fibro- 

 blasts but to fat-cells. They secrete within their cyto- 

 plasm droplets of fat which increase in size and become 

 confluent (Fig. 284). Finally, a single fat globule fills 

 the cell of which the nucleus and cytoplasm are pressed 

 to the periphery. The fat-cells are most numerous 

 along the course of the blood-vessels in areolar connec- 

 tive tissue and appear first during the fourth month. 



FIG. 284. Developing 

 fat-cells, the fat blackened 

 with osmic acid (after 

 Ranvier). n, nucleus; g, 

 fat globules. 



"Pre.Cart: 



Carl. 



CARTILAGE 



Cartilage has been described as developing in two 

 ways: (i) The mesenchymal cells increase in size and 

 form a compact cellular precartilage. Later the hyaline 

 matrix is developed between the cells from their cyto- 

 plasm (Fig. 285, A). The matrix may in this case be re- 

 garded as the ectoplasm of the cartilage cells. (2) Ac- 

 cording to Mall, mesenchymal cells give rise first to an ectoplasm in which 



fibrillae develop. Next, the cells increase in size and are gradually extruded until 



they lie in the spaces of the ecto- 



plasmic matrix (Figs. 2836", 285, B). 



Simultaneously, the ectoplasm is 



converted into the hyaline matrix 



peculiar to cartilage, undergoing 



both a chemical and structural 



change. About the cartilage cells 



the endoplasm produces capsules 



of hyaline substance. 



The interstitial growth of cartilage 

 is due : (i) to the production of new hya- 

 line matrix; (2) to the formation of cap- 

 sules about the cells and their transforma- 

 tion into matrix; (3) to the proliferation of 

 the cartilage cells, which may separate or 

 occur in clusters within a single capsule. 



Perichondral growth also takes place about the periphery of the cartilage and is due to 

 the activity of persisting mesenchymal cells, which, with an outer sheath of connective tissue, 



FIG. 285. Diagrams of the development of carti- 

 lage from mesenchyma (Lewis and Stohr). A, based 

 upon Studnicka's studies of fish; B, upon Mall's Study 

 of Mammals. Mes., mesenchyma; Pre. cart., pre- 

 cartilage; Cart., cartilage. 



