CONNECTIVE TISSUE IN AMPHIBIA 453 



STUDY OF PREPARED MATERIAL 



If sections of a frog embryo, taken shortly after the notochord 

 is formed and before the tail bud stage is reached, are studied, 

 it will be found that the notochord of the embryo is surrounded 

 by a mass of transparent gelatinous material (primitive ground 

 substance). Such a stage is shown in figure 1 which was made 

 from a portion of a frontal section of a 3.2 mm. embryo of Rana 

 palustris. The magnification is 285 diameters. Above a portion 

 of the central nervous system is shown. Extending posteriorly, 

 in the median line, is the notochord imbedded in a gelatinous 

 cell-free matrix or primitive ground substance which extends 

 laterally between the muscle plates in either direction. 



In figure 2 is shown a portion of a transverse section through 

 a 4 mm. embryo of the same species but slightly older than that 

 shown in figure 1. The magnification is 322 diameters. The 

 notochord is shown in the center of the figure. Lying above it 

 is a portion of the ventral wall of the medullary tube. The 

 embryonic muscle cells can be seen lying to the right and left 

 of the notochord. Filling the space between the notochord and 

 the muscle cells is the gelatinous material mentioned above, 

 which constitutes the primitive ground substance. This mate- 

 rial also extends dorsally along the sides of the spinal cord and 

 laterally into the muscle tissues. As is shown in the figure, 

 the ground substance in some regions shows a fine fibrillation 

 running through it. In other regions it appears homogeneous. 



At the stage in embryonic development shown in figures 1 

 and 2, the mesenchyme cells, which later wander all through the 

 cavities of the embryo, are not present in this region in which 

 the primitive ground substance is first formed. The case is 

 clear, therefore, that the ground substance which is here present 

 surrounding the notochord has not arisen by sj^'ncytial fusion 

 of cell cytoplasm. 



The primitive ground substance having been formed as shown 

 in figures 1 and 2, it is next found that, in an embryo of a little 

 later stage, cells begin to detach themselves from the cell masses 

 and wander through it. These cells, when they first appear, 



