326 



INSECTA. 



sides of the oral aperture into the anterior cephalic region, which 

 represent direct prolongations of the primitive swellings (Fig. 160, 

 b\ b" 2 , b z ), and from which are derived those parts of the brain 

 known as the primary ganglia, and named by Viallanes the proto- 

 cerebrum, deutocerebrum, and tritocerebrum. These swellings become 

 early separated as three consecutive brain-segments. Patten (No. 67) 

 has the merit of having first drawn attention to this segmentation. 



2. A large ectodermal thickening lying laterally to the swellings 

 in the cephalic lobes just mentioned (Fig. 160 A, og). This is the 

 rudiment of the optic ganglion, which exhibits in its postero-external 



V 6< ^ 



-- mi 



Fig. 160. — Diagram of the development of the brain in Acilius (after Patten). A, anterior 

 end of the germ-band of an Acilius embryo. B, the same in three-quarter profile. 

 at, antenna; W-, first, Ifi, second, 63, third segments of the brain; i, invagination of the 

 optic ganglion ; P-, anterior, i", posterior portions of the invagination ; I, paired rudiment 

 of the upper lip ; m, mouth ; ind, mandible ; nix', first, mix", second maxilla ; og, optic 

 ganglion ; o</i, first, og 2 , second, og3, third segment of the optic ganglion ; op, optic plate ; 

 1-G, rudiments of the six larval eyes ; I-IV, the four anterior segments of the ventral 

 chain of ganglia ; I, that belonging to the pre-mandibular segment (?) ; II, that belonging 

 to the mandibular segment ; III and IV, those belonging to the first and second maxillae. 



region a semi-circular ectodermal invagination (i, Patten's ganglionic 

 invagination), which yields further elements for increasing the optic 

 ganglion, and corresponds in position with a similar invagination 

 found in the Decapoda (Vol. ii., p. 171) and the Arachnida (pp. 12 

 and 53). 



The part of the ectoderm lying externally to this invagination (Fig. 160^, op) 

 also becomes thickened, increases considerably in extent, and yields at a later 

 period a large part of the cephalic integument and the rudiments of the eyes ; 

 it is therefore known as the optic plate. 



