164 CHAS. B. WILSON. 



nature of muscle tissue as to its origin and functions. And 

 yet, so far as known, the present is the first attempt to work 

 out this development in any pilidium. Even Burger (13), in 

 his excellent monograph, can only say, " The mesoderm gives 

 rise to the gelatine, the star-shaped cells lyiug iu it, and the 

 muscle fibrillse. The latter have doubtless developed out of 

 the star- shaped cells." 



Origin of the Mesenchyme Cells. — Metschnikoff has 

 shown in the genus Linens (35) that the mesenchyme origi- 

 nates from large entoderm cells around the border of the inva- 

 gination, and the same is true in Cerebratulus. When 

 gastrulation first begins, scattered mesenchyme cells separate 

 from the entoderm in the angle between the invagination 

 and the oral surface, and sometimes along the convex in- 

 vagination itself (fig. 72). 



Unlike Linens, there are here two kinds of these cells, 

 readily distinguished by their size (fig. 73). 



The larger ones are given off first, and for a little while are 

 the only ones to be seen ; then both kinds come off together 

 indiscriminately. We will call them macromesencytes and 

 micromesencytes respectively. 



At first the macromesencytes are rounded in form, and 

 float about freely. At this time they are difficult to discern 

 against the background of ectoderm, but they soon become 

 branched, and are then easily distinguished from all other 

 tissue. 



The branches hinder their freedom of motion enough to 

 show that the liquid in which they float is gelatinous rather 

 than watery, and they gradually become fixed in position and 

 function as muscles. While free they exhibit amcBboid 

 movements, similar to those of blood leucocytes; then the 

 cell puts out slender, blunt pseudopodia, which change their 

 position, size, and shape, like those of an amoeba. But they 

 quickly become fixed in position, and are then spun out much 

 farther, with tips so attenuate that it is impossible to tell 

 where they really do end. 



They continue to increase in length until they finally touch 



