442 Avery E. Lambert. 



The fold, together with the optic and cerebral lobes, form a true 

 cerebral vesicle; the roof of the vesicle consisting of the fold, itself, 

 the floor being made up of the ganglionic portions of the cephalic 

 plate. The lumen of this vesicle extends dorsalwards, its opening 

 being in the ventral direction. 



A considerable change has also taken place in the position of the 

 ganglia of the head. As has already been indicated, the concentra- 

 tion of the neural elements, owing to the massing of the neuromeres 

 in the head region, has resulted in pushing some of the ganglia below 

 the surface. As a result of this two parts of the brain may now be 

 distinguished according to the level in which they lie ; first, a super- 

 ficial part which forms the optic portion of the brain, and, second, 

 the large optic vesicle which lies beneath the anterior margin of the 

 cerebral lobes (the oj-gan stratifie of St. Remy), together with the 

 lateral optic vesicles, and a median portion, the cerebral ganglia 

 which have, by this time, come to be overlaid by the optic ganglia. 



The cephalon is divided into two distinct parts by the deepening 

 of a constriction which appears at a somewhat earlier period (cf. 

 Fig. 29) and separates the region in which the anterior optic vesicle 

 lies from the more posterior portions of the cephalic plate (m. /.). 



The chelicerse have been pushed dorsalwards until they lie above 

 the rostrum, which they partially conceal. The segment bearing the 

 pedipalps now lies immediately beneath the stomodseum. 



The neviromeres of the thoracic segments appear as oblong masses 

 of nerve tissue, each segment being separated from the one anterior 

 to it by a small, but distinct, furrow. In this the Araneina repeats 

 the condition which Patten (35) found in the scorpion, except that 

 the neuromeres, themselves, fail to show any indication of a division 

 into two parts. 



At this stage in the development of the embryo the mittelstrang 

 appears as a shallow, longitudinal furrow between the two halves 

 of the nerve cord (ec. f.). This furrow is not extensive. Its point 

 of origin, in the region of the stomod^um, is hidden by the lower 

 margin of the rostrum. It ends in the structure which was noted in 

 the previous stage as the "ectodermal pit" (Figs. 28, 29, and 30, 

 e. p.), which is now^ located between the neuromeres of the first 

 thoracic segment. 



