The Development of Theridiuin. 301 



ing the egg surface draw with them the intravitellar cytophismic 

 network. 



The thirty-two-cell stage (11. Lours) is reiDresented in Fig. 7, a 

 reconstruction from serial sections (made like Figs. 3 and 5) ; some 

 of the sections were broken and only thirty nuclei could be found. 

 None of the cleavage cells have yet reached the egg surface. Inequal- 

 ity in the rate of cell division is commencing, for while a few nuclei 

 are in the rest stage most of them are in mitosis. 



A stage of 12 hours with from sixty to seveJity cells is shown in 

 Fig. S, a surface view of one hemisphere of the egg. Few of the 

 cells are quite at the surface, but all of them close to it; this is the 

 earliest cleavage stage at which the cleavage cells can be distinctly 

 seen on whole mounts. The heavy line, Cav. B, marks the boundary 

 of the central fluid cavity. The cells, still without separating walls, 

 are connected by delicate branching processes. 



The next stage is that of the early blastoderm with all the cleavage 

 cells at the surface. The one figured is of the age of ll^/^ hours, but 

 from this time on the age is no true gauge of the stage. A surface 

 view of one hemisiihere is drawai in Fig. 10, and a segment of a single 

 section in Fig. 9. The cells are fairly evenly distributed on the 

 periphery, not noticeably more numerous on any one pole than at 

 another, and number about 140. It is to be noted that no cleavage 

 cells remain within the yolk. 



The movement of cells toward the surface is a peripheral move- 

 ment of the whole intravitellar cytoplasm, so that when the cell 

 bodies have merged with the blastema cytoplasmic threads penetrate 

 for only a short distance into the yolk (Fig. 9). 



Fig. 11 exhibits on surface view a hemisphere of an egg of 14 hours, 

 a stage of about 300 cells. The cell bodies are beginning to project 

 slightly above the surface of the egg. 



• The next stage shows the beginning of the segregation of the 

 ventral germ disc or embryonic region (ventral plate), which is 

 characterized by a greater number of cells, while the extraembrj'-onic 

 (dorsal) area possesses fewer cells. PI. II, Fig. 12, is a lateral sur- 

 face view showing 173 cells on this side, the total number of cells 

 being about 250; and Fig. 13 is a section of the germ disc. ISTow 



