Proccphalic Lobes of Eperia Cinerea. 431 



In this stage we find that the middle region of the cephalic plate, 

 which lies close to the anterior margin, has become slightly raised 

 bj the thickening of the ectoderm. This can be readily determined 

 by manipulating the egg with needles so that the structure can be 

 clearly seen by reflected light. The prominence formed by this ele- 

 vation of the ectoderm is the foundation of the future rostrum, or 

 upper lip (ros.) 



Stage II. Fig. 20. — The second embryonic stage is marked by 

 a slight advance in the development of the cephalic plate. The num- 

 ber of body segments may have increased, but there is no fixed 

 number which can be regarded as characteristic of this period. The 

 rudiments of the thoracic appendages have also become more promi- 

 nent. Of the head appendages, the pedipalps especially have length- 

 ened, and now show a tendency to bend toward the median line, 

 a position which is characteristically maintained by all the append- 

 ages except the chelicer?e up to the time of hatching. 



A characteristic feature of this stage, to which attention should 

 be called, is the conspicuous furrow which separates the cheliceral 

 segment from the pre-cheliceral part of the cephalic plate. It thus 

 becomes evident that all of the appendage-bearing segments belong 

 to the same series and can not be disting-uished from one another 

 in the earlier stages as the use of the terms "thoracic" and ''cephalic" 

 appendages w-ould seem to indicate. The union of the first two 

 appendage-bearing segments with those portions of the embryonic 

 plate which give rise to the cerebral structures is a distinctly later 

 event. 



The development of the lateral areas of the cephalic plate has 

 continued until the ectoderm is deeper in this region than in any 

 other portion of the plate. The appearance of the lateral tracts in 

 surface views as a considerably darker area is due to the greater 

 thickness of the ectoderm, and the closely crowded nuclei of its cells 

 (o. gl.). The rudiment of the rostrum shows a slight advance by its 

 more marked separation from the adjacent parts {ros.). 



Stage III. Fir/. 21. — In this stage a distinct condition of advance 

 is to be recognized in the development of the head region. The lateral, 

 or optic, area is more certainly differentiated from the medially 



