454 GEORGE A. BAITSELL 



are for the most part more or less rounded in shape and contain 

 a considerable amount of yolk. As they move through the 

 ground substance they become amoeboid and also in some cases 

 spindle-shaped; a form which is regarded as being typical of the 

 so-called connective tissue fibroblasts. It is frequently possible 

 to trace the movements of the cells by the altered appearance 

 of the gelatinous ground substance. 



In figure 3 is shown a portion of an unstained transverse 

 section through a 9 mm. embryo at a magnification of 322 

 diameters, at the stage when the cells are starting to move into 

 and through the ground substance, as described just above. In 

 this figure a portion of the notochord is sho^Mi with embryonic 

 muscle tissue lying on either side. Both the notochord and the 

 muscle tissue are imbedded in a matrix of the ground substance 

 which is especially heavy in the region just below the notochord. 

 From this region, it spreads to the right and the left surrounding 

 the bundles of muscle fibers and extending to the body wall 

 on either side. At the stage shown numerous cells are to be 

 seen in the portion of the ground substance which lies below the 

 notochord. These cells vary considerably in shape; some are 

 rounded, while others have assumed a spindle-shape. 



In figure 4 is shown a portion of a transverse section through 

 the tail region of an 11 mm. embryo which was preserved in 

 Zenker and stained with Mallory's stain. The magnification 

 is 760 diameters. The portion of the section shown in this 

 figure lies just below the notochord. At this point there is a 

 considerable area of the primitive ground substance. It is 

 surrounded ventrally and laterally by developing muscle tissue 

 and dorsally by the notochord. In this figure it is desired to 

 lay particular emphasis upon the relations shown between the 

 ground substance and the cells which are present in it. The 

 cells which have wandered into this region have for the most 

 part become amoeboid or spindle-shaped in type and they 

 possess long cytoplasmic processes w^hich stretch out in various 

 directions through the ground substance. The figure shows that, 

 in a preparation of this kind which has been properly stained with 

 the Mallory connective tissue stain, the differential color reaction 



