MUSEUM OF COMPARATIVE ZOOLOGY. 205 



cells bordering the cavity of the intestine ; elsewhere it occurs only as 

 scattered granules. 



At this stage two plates of mesoderm (Fig. 2, la. msWrm.) extend out 

 laterally, one on each side of the chorda, and pass ventrally around the 

 mass of yolk cells to be united in the median line below. Each plate is 

 thickest (Figs. 1,3, at la. pr'vr.) next the notochord ; as it passes out- 

 ward, it becomes thinner. Before the ventral surface of the embryo is 

 reached, it is reduced to a layer two cells thick, representing the somato- 

 pleure and splanchnoplenre (so'plu. and spVplu.) of this region. The cells 

 of the mesoderm are in general smaller than those of the yolk-entoderm. 

 The yolk spherules which they contain are also somewhat smaller than 

 those in the entoderm. Pigment is rarely present except in the form of 

 scattered granules. 



In the foregoing account of the relations of the germ layers the de- 

 scription refers in the main to She typical condition, realized in the 

 middle trunk region ; in this and in subsequent stages modifications 

 occur in the head and tail regions. These special conditions are of no 

 consecpience for the present purpose. 



There are certain histological characters, to which allusion has already 

 been made, which may serve as criteria for distinguishing the germ 

 layers. The most satisfactory of these is the size of the yolk spherules. 

 As I have indicated, the spherules are largest in the entoderm and 

 smallest in the ectoderm ; in the mesoderm they are of an intermediate 

 size. Measurements of spherules from the three layers in the region of the 

 future pronephros gave the following results : entoderm, mean diameter 

 of spherules, 8 /j. ; mesoderm, mean diameter, 5 /x ; ectoderm, diameter 

 rarely exceeds 3 /a. Excluding the head and tail regions, these dimen- 

 sions represent, I believe, fair averages for the whole body. The dis- 

 tribution of pigment affords another criterion for distinguishing the layers. 

 In the superficial ectoderm, the pigment (Figs. 3, 7) is massed along the 

 external surface of each cell. In the deep ectoderm, it is present in con- 

 siderable quantity, but is scattered throughout the cell. Except in cer- 

 tain specialized regions, there is little pigment in either mesoderm or 

 entoderm. I have also noted the differences in the mean sizes of the 

 cells : the yolk cells are in general the largest, and those of the ecto- 

 derm the smallest, the mesodermal cells being of intermediate size. 

 The great variability of this character prevents its having much weight, 

 however, in determining to which of the three layers a given group of 

 cells belongs. 



I shall now consider in greater detail some of the modifications which 



