Procephalic Lobes of Epeira Cinerea, 439 



also advances in the medio-posterior direction, its increased growth 

 making its character somewhat more apparent. 



The pedilaps and chelicerse have continued to enlarge ; a mandibu- 

 lar segment being formed by the growth of the l)asal joint of the 

 pedipalps in the median direction. This segment is separated from 

 the main portion of the pedipalp by a deep constriction at the point 

 where the appendage and the coxa are united (md. pdp.). 



It is to be observed, in all stages, that the development of the 

 cephalic plate does not, in every case, keep pace with the progress 

 made by the remaining parts of the embryo, tliis body frequently 

 showing a state of development which is in advance of what may be 

 regarded as the normal condition of the cephalic plate. Occasion- 

 ally embryos are found in which the two halves of the brain do not 

 keep pace with one another. Continued growth, in most cases, equal- 

 izes these irregularities ; although, in rare cases, the divergence 

 becomes so great that an abnormal structure of the embryo is the 

 result. 



The most frequent of these al)normalities to be met with is the 

 double ombr)^o, in which the anterior portion of the germ band grows 

 in two directions, two cephalic plates being formed. A single caudal 

 plate serves for the two embryonic bands which unite in the thoracic 

 region. 



Stage VII. Fig. 27. — From this point on the coalescence of the 

 ganglionic elements of the cerebral lobes proceeds with great rapidity 

 and regularity. The lips of the anterior optic invagination have 

 closed so that the groove can l)e made- out with difficulty in surface 

 views. The closure is apparently due, as are nearly all the important 

 changes which are taking place in the form of the cephalic plate, 

 to the forward migration, and to the general concrescence, of the 

 neuromeres. This closure is more apparent in the lateral parts of 

 the invagination, being less complete in the median portion of the 

 lobes. 



The anterior optic invagination has increased in depth to the 

 extent that it now forms a lar^x^ vesicle which has come to lie under- 

 neath the anterior margin of the cerebral lobes. The anterior, or 

 dorsal, lip of this vesicle is directly connected with the optic plate 



