THE FORMATION OF THE ORGANS — THE NERVOUS SYSTEM. 191 



The cerebral ganglia mighl al once be referred back to the apical 

 plate of tltc Trochopk&re, were it not for the fact, about which authors 

 seem to be fairly unanimous, that the ganglia here appear in the 

 form of two distinct thickenings of the ectoderm (Fig. 68, eg) winch 

 only unite later by the formation of the cerebral commissure. P. 

 Sarasin, indeed (No. 101), has stated for Bythinia, that the two 

 ectodermal thickenings at first are connected by a median ectodermal 

 growth, and thus (in their origin at any rate) suggest a common 

 rudiment, but this method of formation, which in itself is very 

 probable, has been directly denied, not only for Bythinia but for the 

 related form Paludiwx (v. Erlanger, Xos. 27 and 28). The two 

 thickenings belong to the velar area, lying laterally in it in front of 

 the mouth. Even if the cerebral ganglion forms with the help of 

 an invagination, its rudiment is paired. In the Pulmonata, in which 

 this method of formation of the brain is best known, there are at 

 first the two ectodermal thickenings which here also yield the 

 principal mass of the cerebral ganglia in the usual way. Then, 

 when these are already partly detached from the ectoderm, a 

 depression of the ectoderm occurs at the lower edge of the posterior 

 tentacles ; this becomes continually deeper, and thus forms a tube 

 (Sarasin's cerebral tubes). According to P. and F. Sarasin, in 

 Helix (Acavus) Waltoni, there are two such cerebral tubes on each 

 side (Fig. 87 A, <-t) while, in Limax, only one is found on each 

 side (Henchman, No. 42 ; F. Schmidt, No. 110). The blind ends of 

 the cerebral tubes become applied to the rudiments of the cerebral 

 ganglia which have become further differentiated (Fig. 87 A, ct, eg), 

 and fuse with these to form that part of the brain which is known as 

 the accessory lobe (Fig. 87 B, ct). They then become abstricted 

 from the superficial epithelium. Their lumina can still be recognised 

 as fissures (Fig. 87 B), but these soon entirely disappear, the forma- 

 tion of the brain being thus practically completed. A differentiation 

 of the principal part of the brain into ganglionic cells and fibrous 

 tissue had already taken place. 



Sarasin's observations with regard to the cerebral tubes, which were on the 

 whole confirmed by the researches of F. Schmidt and Henchman, afford an 

 explanation of the apparent contradiction involved in the two views of the 

 origin of the cerebral ganglia, which were derived by one author by invagina- 

 tion, and by another in the same or related forms by delamination. Both 

 these views are founded on fact, each being observed at a different stage of 

 development. In this respect, those forms in which the brain arises as two 

 depressions of the velar area, as is the case, according to Fol, in the Pteropoda, 

 require more careful investigation. The two invaginations are no doubt 



