Procephalic Lobes of Epeira Cinci'ea. 437 



After the uniou of the two halves of the dorsum their continued 

 growth produces a pressure upon the most posterior of the abdominal 

 neuromeres which causes them to shift anteriorly ; a process which 

 results in the shortening of the entire ventral surface of the embryo, 

 the shifting neuromeres becoming associated with segments anterior 

 to those with which they originated. 



The results of the forward shifting of the neural elements are also 

 evident in the cephalic plate. The cheliceral segment is among the 

 first to respond to this crowding of the neuromeres in a forward 

 direction, so that it becomes closely pressed against the segment in 

 front, thus entirely obliterating the furrow which separated these 

 two segments in the preceding stages. 



The anterior groove is not greatly influenced by this pressure from 

 the posterior direction in this stage, the lips of the invagination 

 being quite widely separated. The optic and cerebral ganglia are 

 quite distinct, and are easily distingiiished in surface views. The 

 examination of these elevations of the ectoderm in the cephalic region 

 is facilitated by rolling the egg from side to side, a process which 

 brings the contours into more perfect relief. 



The rostrum (ros.) which had shifted, in the preceding stage, 

 from its first position near the anterior margin of the head, has 

 moved still farther in the posterior direction. 



One of the most marked advances which the embryo has made in 

 this stage, is the beginning of the structure which Patten describes 

 for the scorpion, and to which he has given the name '^cephalic fold" 

 or "hood." As the growth of the cephalic lobes progresses, their 

 margins, Avhich consist of the thickened limb of ectoderm, the optic 

 plate, is raised slightly and turned in the medio-postcrior direction. 

 The fold (o. f.) thus formed continues to advance in this direction 

 as the development of the embryo continues, bearing the inverted 

 optic plate on its margin. In this stage the fold progresses until it 

 has come to lie above the opening of the anterior groove, and the 

 more anterior portions of the cephalic lobes. 



Stage VI. Fig. 26. — The parts of the cephalic lobes which, in the 

 preceding stages, have been quite distinct, are now beginning to lose 

 their identity by uniting with one another. The cerebral ganglia are 



