The Development of Theridiiim. 305 



Besides fixing the axes of the embryonic region the development of 

 the posterior cumulus determines an important boundary. Just 

 anterior to the posterior cumulus the cells of the germ disc are 

 thinner than elsewhere {Th. Ah, Figs. 41a, PI. Ill; 42a, PI. IV); 

 this will become the boundary of the cephalothorax and abdomen. 



We may next consider the more important processes that are 

 progressing at the anterior cumulus. This has become larger and 

 more irregular in outline {Cum. A, Fig. 29, PI. II; 32, 33, PI. Ill; 

 44, PI. IV), and is slightly elevated above the surface of the germ 

 disc (Fig. 26, PI. II). It maintains its pitlike gastrocoel (Gast., Figs. 

 27, 28, PI. II; 37b, 38 A, B, PL III), that closes at the stage illus- 

 trated by Figs. 40A, B, 41A, C, PI. III. It is still somewhat variable 

 in position, though always behind the center of the germ disc. At the 

 stage of 0OY2 bours it is shown on median section in Figs. 27 and 28, 

 PI. II ; the invaginated cells have increased in number and size, and 

 those in contact with the yolk have become greatly branched and 

 coarsely vacuolar with ingested yolk particles. These large cells are 

 vitellocytes, and they undergo a continuous emigration from their 

 point of origin, which is in part a movement into the yolk, but to 

 greater extent a passage from the periphery of the cumulus outward 

 between the yolk and the germ disc; this is what causes the outline 

 of the cumulus to become larger and more irregular. At a little later 

 stage this wandering becomes more pronounced, as shown in the 

 middle of Fig. 31, PI. Ill (a cross section through the anterior edge 

 of the anterior cumulus), and Fig. 30a (where only a lateral edge of 

 this cumulus is cut). 



A more advanced stage of the anterior cumulus, 37^/2 hours, is 

 represented in Figs. 34-39, PL III. Fig. 34 is a ventral surface 

 view of a germ disc containing 1222 superficial cells; the shaded 

 region marks the area where vitellocvtes lie, in a broad band extend- 

 ing from a little anterior to the centre of the germ disc back to the 

 posterior cumulus {Cum. P) ; the two cumuli have become intercon- 

 nected by vitellocytes arising mainly from the anterior one. Fig. 38a, 

 an oblique longitudinal section of the whole germ disc, and Fig. 38b, 

 an enlarged drawing of the anterior cumulus alone, show the gastro- 

 coel to be a pit bounded immediately by a group of large unbranched 



