Procephalic Lobes of Epeira Cinerea. 443 



Stage X. Fig. 31. — Progress toward tlie adult condition is well 

 marked in the embryos which have been selected to represent this 

 stage. The brain is seen to consist of two distinct parts, the pro- 

 cephalon, or supra-oesophageal (Fig. 34, hv.) mass, and the post- 

 cephalon, or snb-cesophageal mass (Fig. 34, s. o, g. 1-12). The lobes 

 of the pro-cephalon do not become so perfectly fused that they lose 

 their identity until a little later. The ganglia of the post-cephalon, 

 although pressed closely together, are never fused. 



Practically the entire area of the cephalic plate is covered by the 

 optic fold, which now extends to, and partially covers, the base of the 

 chelicerse. The parts of the fold which bear the rudiments of the 

 anterior median eyes have come together in the median line. The 

 ectodermal thickenings wdiich form the rudiments of the lateral eyes 

 (Z. e.) are located on the lateral margins of the head, not far from 

 the bases of the chelicerse. 



The ganglia of the thoracic segments have crowded forward until 

 they are packed closely together just beneath the stomodaeum ; the 

 remainder of the thorax beneath the cesophagiis being filled with 

 ganglia consisting of the neuromeres of those abdominal segments 

 which have shifted forward into this position. 



The formation of the mittelstrang is completed by having the lips 

 of the ectodermal furrow unite to form a tube which lie? just beneath 

 the ectoderm between the neuromeres of the first abdominal segment. 

 This tube ends, posteriorly, in the region in which the ectodermal pit 

 was located. In the method of their formation the furrow and the 

 pit resemble very closely the infoldings which develop, in part, into 

 the "limatochord," and, in part, into the intra-ganglionic commissures 

 which Patten (35) mentions in connection with his description of 

 Buthus. 



In front of the antero-lateral margins of the head the ectoderm is 

 raised into a slight fold (a. /.). This fold is probably to be compared 

 with the so-called amniotic fold of insects. 



Large cells with peripherally located nuclei, which are present 

 near the margins of the embryonic plate in nearly all of the preced- 

 ing stages, appear, in this stage, to he gradually sinking into the yolk. 

 The precise nature of these cells is doubtful. They have been regarded 



