Harry Lewis Wieman 195 
a certain structure are not confined to any one stage. In the following 
account cells are said to belong to a certain stage because they were 
first noticed in that stage, although they may be, and frequently are, 
seen in sections from tissue several days older. 
Thirty hours (15-somite stage).—Cells of this stage exhibit in longi- 
tudinal section a characteristic short, broad, cylindrical form (Fig. 1). 
The ends of the cell taper rather abruptly from the center. The nucleus 
(Fig. 1,d) is large and oval; its short diameter being but slightly less 
than the diameter of the cell. Large chromatin masses (Fig. 1, b) may 
be seen as heavily stained, irregularly shaped clumps, and a very fine 
network traverses the nucleus. 
The cytoplasm exhibits a pronounced reticular structure, with large 
and irregular meshes. At the intersections of the threads of this network, 
the staining is somewhat heavier, marking off these parts more distinctly 
than the rest of the reticulum. 
Fig. 2 is a cross section through the region of the end of the nucleus. 
The cytoplasm shows the same reticular structure, the nucleus appearing 
oval in outline. 
Cells of this general description are typical of the heart tissue in 
its very earliest state of formation. They can be readily distinguished 
from the unmodified mesenchyme cells, in that they do not show the 
branched structure of the latter. 
Seventy-two hours (3-day stage).—Up to this period in its develop- 
ment, the cell differs but slightly from the description given above. At 
this time, however, several changes are to be noticed. The cell has become 
larger and the tapering ends have increased in length (Fig. 3). The 
points of intersection of the threads of the cyto-reticulum are more 
distinctly marked than before by accumulations of heavily staining 
material, spherical in form (Fig. 3,a@). These are also shown in cross 
sections (Fig. 4,a). The nucleus is oval in form though somewhat 
smaller than. in earlier stages. The chromatin masses (Vig. 3,0; Fig. 
4,b) have decreased in size. 
Ninety-six hours (4-day stage) —Fig. 5 represents a longitudinal 
section of a cell of this period. The cell has increased enormously in 
size, especially in its longer diameter. ‘The deposits on the cyto- 
reticulum are larger and stand out very clearly and distinctly (Fig. 5, a). 
The meshes of the cyto-reticulum are still very irregular in form. The 
nucleus does not seem to have undergone the same increase as the cyto- 
plasm, and its chromatin masses have become smaller and more evenly 
distributed (Fig. 5,b; Fig. 7,6). Fig. 6 shows a cross section above 
