THE MESENCHYMA. 



89 



the anlages* of the muscles of the eye, and these are, by hypothesis, homologous 

 with the cells of the walls of the head-cavities in the lower vertebrates, which cells 

 produce the muscles of the eye. 



The Mesenchyma. 



By the term mesenchyma we designate the whole of the mesoderm of the em- 

 bryo, except the mesothelial lining of the coelom. When fully differentiated his- 

 tologically, it consists of more or less widely separated cells, connected with one 

 another by intervening threads of protoplasm, which form a network between the 

 cells (Fig. 49). The remaining space is filled by a homogeneous structureless ma- 

 trix or basal substance. It gives rise to a large 

 number of adult tissues, as shown in the table 

 on page 19. 



In the early development, or histogenesis, of 

 the mesoderm we can distinguish four stages: 

 first, that of distinct cells; second, the forma- 

 tion of the cellular network; third, the forma- 

 tion of the mesothelium; and, fourth, the differ- 

 entiation of the mesenchyma. The first stage is 

 known chiefly through observations on the early 

 stages of elasmobranchs, reptiles, and birds. In 

 these types the first cells which are delaminated 

 from the entoderm to form the anlage of the meso- 

 derm, are of quite large size and lie between the entoderm, or yolk, and ectoderm, 

 and are without connection with one another. The number of mesodermic cells in- 

 creases both by the multiplication of the cells already delaminated and by the addition 

 of others from the entoderm. Whether this stage occurs in mammals or not, we do 

 not know at present. In the second stage the primitive cells are found to have ac- 

 quired connection with one another, the protoplasm of one cell uniting by a process, 

 or prolongation, with the protoplasm of another cell, and so on until the whole 

 tissue becomes a network. When the primitive streak has been formed in the 

 mammalian blastodermic vesicle we find the mesoderm in this condition. The third 

 stage is brought about by the development of the coelom as above described. The 

 coelom is bounded by the undifferentiated mesoderm. To produce the fourth stage, 

 single cells leave the primitive mesodermic layer by migrating out of it on the 

 side away from the ccelom. The cells left behind are ultimately reduced to a sin- 

 gle layer, the permanent mesothelium. The emigrant cells constitute the mesen- 

 chyma and are found to be connected both with one another and with the meso- 

 thelial cells by protoplasmatic processes, but they do not lie close together, as in 

 the epithelium, so that there is a considerable, though variable, amount of inter- 



* The anlages may be seen in a pig embryo of 10 mm. between the jugular vein and the internal carotid 

 artery as a group of embryonic cells quite distinct from the surrounding mesenchyma. 



J8 

 FIG. 49. CHICK EMBRYO OF THE THIRD 



DAY. 



Mesenchyma from near the otocyst. A, cell 

 in mitosis. 



