THE MESENCHYMA. 83 



The Coelom of the Head. No adequate investigation of the early stages of 

 the mesoderm in the head of amniota has yet been made. We know, however, 

 that in the lower vertebrates there appear at least three distinct cavities re- 

 sembling portions of the true coelom and bounded by epithelial cells, similar to 

 the mesothelium in character. These cavities are generally regarded as portions 

 of the true coelom, and by many writers have been interpreted as true primitive 

 segments. But this interpretation is not yet beyond doubt. The largest of 

 these cavities is called the mandibular, because it has a prolongation which ex- 

 tends into the mandible of the young embryo. In front of it is the first or pre- 

 mandibular cavity, which is much smaller, and behind it is the third or hyoid 

 cavity, which is intermediate in size between the first and second. The head- 

 cavities are best known in the elasmobranchs. They have also been found 

 clearly developed in reptiles and certain birds. In mammals no actual cavities 

 have been recorded. There are found 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 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 

 embryo, except the mesothelial lining of the ccelom. When fully differentiated 

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

 one another by intervening threads of protoplasm, which form a network be- 

 tween the cells. The remaining space is filled by a homogeneous structureless 

 matrix or basal substance. It gives rise to a large number of adult tissues, as 

 shown in the table on page 35. 



In the early development, or histogenesis, of the mesoderm we can distin- 

 guish four stages: First, that of distinct cells; second, the formation of the 

 cellular network; third, the formation of the mesothelium; and, fourth, the 

 differentiation 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 mesoderm, 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 increases both by the multiplication of the cells already de- 

 laminated, 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 



* 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. 



