33-i The Development of the Connective Tissues 



transparency. The main body of the cell mass has become decidedly 

 multipolar in character and if anything, smaller than that in the 

 embryo 3 mm. long. The yolk discs have largely disappeared while 

 those remaining have become more transparent. On account of this 

 change the nuclei are more distinct than in the earlier stage. The 

 main cell body still contains many pigment granules. From each of 

 the many poles of the cell fine threads of protoplasm arise, which 

 divide once or twice, and anastomose into the same kind of threads 

 from neighboring cells. In other words the multipolar cells form a 

 complete syncytium. What now forms the main cell body gradually 

 becomes a nodal point in an older stage of the syncytium. In this 

 embryo we have mesenchyme pure and simple in the tail and a com- 

 plete syncytium formed by the mesenchyme around the anterior end 

 of the chorda. Between these there exist of course all intermediate 

 stages. 



In an embryo 6 mm. long no very great change has taken place in the 



development of the sj^ncytium (Fig. 3). The cells in the tissue around 

 the anterior end of the chorda appear much as in the earlier stage, 

 with the exception that the protoplasmic bridges between the cell 

 bodies are somewhat thicker and have a slight fibrillar structure, form- 

 ing the first exoplasm. There are also some vacuoles in each process 

 which indicate that an individual bridge is widening and breaking up 

 into a number of bundles. The yolk discs and pigment granules are 

 about as numerous and as definite as in the embryo 4 mm. long. The 

 mesenchyme of the tail is now in the form of a complete syncytium on 

 both its dorsal and ventral sides. 



In another embryo slightly larger than the one just described and 

 just before the mouth breaks through, the connective-tissue syncytium 

 is of different arrangement in different portions of the body. Around 

 the anterior end of the chorda the protoplasmic filaments of exoplasm 

 form a dense network of fibrils a little more advanced than in the 

 embryo 6 mm. long. They are now arranged as bundles between which 

 there are numerous spaces. The endoplasm around the nucleus, in- 

 cluding its transparent yolk discs and pigment, is spreading over the 

 fibrillar network of exoplasm. It appears as if the main mass of 

 endoplasm around the nucleus is being drawn upon to form more of 

 the fibrillar exoplasm of the syncytium in its further growth. The 

 nuclei can now be plainly seen lying upon or within the dense masses 

 of fibrillar exoplasm of the syncytium. 



In front of the brain the cells of the mesenchyme are spindle-shaped 



and run out into fibrils of thicker bands of protoplasm which form a 



