The Development of Theridiiim. 317 



become a hemispherical cup, seen on longitudinal section in Figs. 

 75b and 80b, and on transverse section in Fig. 7Cb; it is immediately 

 lined by rostral mesoblast. Its bordering lip is circular, an ecto- 

 blastic ring into which projects rostral mesoblast (Fig. 76b). 



The cerebral ridges have become more complicated, and on sur- 

 face views (C'e. R., Figs. 71, 72, PI. VI; 70, PL VII) each is seen 

 to have grown from the stomodaeal region postero-lateral to the 

 antero-lateral vesicle {A. L. 7.). Posteriorly (toward the stomodaeal 

 side) each ridge is bounded by the fovea {Fov., Figs. 72, 73, PI. VI; 

 79, PI. VII). The fovea has become a deep groove, deej)ening first 

 mesially, and successive stages of its insinking are shown in Figs. 

 75b and 80d, PI. VII {Fov). The cerebral ridge is the commence- 

 ment of the cerebral ganglion, and the fovea, which is its posterior 

 bordering groove, the ventricle of this ganglion. At this stage the 

 cerebral ganglia are therefore invaginating and sinking below the 

 surface. 



Lateral and somewhat posterior to each of the preceding ganglionic 

 anlages lies a still deeper and more complicated pit, the antero-lateral 

 vesicle (A. L. Y., Figs. 71, 72, PL VI; 79, PL VII). The cavity of 

 this vesicle is mesially continuous with the fovea (Fov.). Where 

 this pit is deepest {A. L. V., Fig. 80c, PL VII) its ectoblastic wall 

 {Ect.) is greatly thickened. When the antero-lateral vesicle is looked 

 at from the surface in a favorable position (as in Fig. 71, PL VI, 

 A. L. y.), and before it has joined with the postero-lateral vesicle 

 (P. L. v.), it may be described as bounded laterally by a semicircular 

 ridge and mesially by an elevated prominence {Ft'.). The promi- 

 nences of both right and left vesicles are lettered Pr. in the transverse 

 section represented in Fig. 76a^ cut anterior to the rostrum; each 

 prominence is a greatly heightened ridge, with folded outer surface, 

 lying between the fovea and the pit {A. L. F.) of the antero-lateral 

 vesicle. This relation is somewhat difficult to describe, but may be 

 understood by comparing the surface view of Fig. 71, PL VI, with 

 the section. Fig. 76a, PL VII. These projecting knobs of the median 

 walls of the antero-lateral vesicles might suggest, from the examina- 

 tion of surface views alone, that they are additional cephalic append- 

 ages; but that they cannot be, for each is simply an ectoblastic 

 elevation that has no special coelomic sac. 



