Procephnlie Lolws of Epeira Cinerea. 429 



While this lengthening of the ventral plate is going on, some very 

 important changes are occurring in its structure. The mesoderm 

 cells which are, at first, spread out in a thin sheet under the ectodenn, 

 now begin to arrange themselves in ridges which lie transversely 

 across the band. (Fig. 8; seg.) These ridges, or primitive seg- 

 ments, are separated by furrows, each furrow being bridged by a 

 thin sheet of ectoderm. A widening and lengthening of the cephalic 

 plate is to be noted at this time, due to an increase in the number 

 of its cells. 



As the growth of the ventral plate continues there is a continued 

 increase in the number of its segments. After seven or eight of 

 these segments have been formed, small, rounded protul^e ranees appear 

 on the first four. These are the rudiments of the thoracic append- 

 ages. (Fig. 9; ap.) Locy states that the fourth thoracic append- 

 age is the first to appear in Agalen<i; the third, second, and first, 

 appearing in order. In Epeira, also, the appendages follow this order 

 in their appearance. 



After the thoracic segments are formed two appendage-bearing 

 segments arise in the posterior region of the cephalic plate. The first 

 of these to appear is the segment which bears the pedipalpi, the second 

 being the segment of the chelicerfc. 



When the appendages of the head and thorax have arisen, and the 

 ventral plate has lengthened until from ten to twelve segments can 

 be counted, it may be fairly said that the first embryonic stage has 

 been reached. 



IV. Development of tpie Peocephalic Lobes. 



Stage I. Fig. 19. — In its general development Epeira cinerea 

 conforms very closely, in most points, to the development of other 

 members of the Araneina as described by the authors already cited. 

 Consequently, in the following study I shall confine myself to a 

 discussion of the origin and development of the procephalic, or as 

 they are frequently called, the cerebral, lobes and their associated 

 sense organs. 



A^Tlile free use has been made of sections for the ])urpose of con- 

 firming results, the major part of the observations recorded were 



