The Development of Theridium. 303 



on cross sections. From the start this cmnulus shows a pit-like de- 

 pression, not a linear groove, the gastrocoel {Gast., Fig. 16) ; this 

 deepens as the gastriilation proceeds. Fig. 19 is a cross section of 

 a stage in which twelve to thirteen cells have pushed in ; Figs. 20 and 

 21, of one with about twenty-five invaginated cells, and Figs. 22-25 

 of a still later stage, when the germ disc contains more than 500 

 cells. This gastrulation process is a double one: (1) By vertical 

 mitoses of cells of the region of the cumulus, and (2) by inrolling of 

 cells, else a gastrocoel could not be formed. The pressure of the yolk 

 causes the gastrocoel to remain a rather shallow pit. 



All the cells of the germ disc retain short processes penetrating 

 into the yolk, these being the last remnants of the former intravitellar 

 mesh. But those cells that invaginate develop longer processes and 

 become more irregular in form (Figs. 17, 19, 21, 24, 25). The 

 innermost of the invaginated cells begin to separate from the others 

 and, as the earliest yolk cells, vitellocytes, to wander into the yolk 

 (Figs. 21, 24, 25). These cells are larger than those on the surface 

 of the germ disc, they are assimilating yolk more rapidly, and for the 

 most part possess also larger nuclei. 



At its periphery the germ disc is not sharply delimited from the 

 extraembryonic area (Figs. 15, 22) ; as its cells increase by mitosis 

 they become more crowded together, whereby their cell membranes 

 appear more distinct, their intercellular processes shorter and thicker, 

 and they come to project more above the surface of the yolk. Where 

 these cells are most numerous they have completely merged with the 

 blastema, consequently this blastema remains distinct from cell 

 masses only at the periphery of the germ disc and in the extra- 

 embryonic area (Fig. 16). 



The extraembryonic cells are not dividing, are widely separated 

 from each other, membraneless and much branched; they stain less 

 deeply than those of the germ disc. When a yolk extraovat is pro- 

 duced by the action of the fixative it is formed in the extraembryonic 

 region. 



3. The Later Pari of the Gastrulation. 



The stages now to be described are found in eggs from 30 to 55 

 hours, with from 1000 to 1500 superficial cells on the germ disc. 



