54 



THE SUBDIVISIONS OF THE BRAIN. 



A pair of small appendages, ro, lie near the first metamere. Later, they fuse 

 to form the labrum. The second metamere has no appendages. The third one 

 is closely associated with the antennae. 



From the upper or neural surface of the first ( ?) and second forebrain neuro- 

 meres, are developed dense masses of small cells, with deeply stained nuclei, that 

 give rise to the characteristic mushroom bodies of insects. They may be recog- 

 nized in apparently all classes of arthropods, attaining enormous size in the ants, 

 bees, wasps and spiders, and reaching extraordinary dimensions in Limulus. 

 In structure and function (Limulus), they are true coordinating centers, and 

 are to be regarded as the earhest stages in the phyllogenetic development of the 

 cerebral hemispheres of vertebrates. 



Fig. 44. — Sagittal section of a primitive vertebrate embryo, showing the relation of its principal organs to those in 



the arachnids; schematic. 



In the arachnids, the cephalic lobes differ from those of Acilius: a. in the in- 

 distinct segmentation of the optic plate, b. in the relatively late appearance of the 

 segmental sense organs (median, and lateral, eyes, and olfactory organs), and c. 

 in the peculiar character of the first metamere. Other differences appear later, 

 as we shall presently indicate. 



In the scorpion (Fig. 15), which may be taken as the type, the first metamere 

 is never divided into neuromere, optic gangHon, and optic plate, but forms at the 

 outset a deeply grooved transverse band, ol.o. The walls of the infolding contain 

 minute, deeply stained nuclei, that make it very conspicuous, both in sections and 

 surface views. The band marks the primitive anterior end of the brain and is the 

 anlage of the olfactory lobes. The infolding deepens, at first more rapidly at 

 either end, and ultimately carries the whole lobe below the surface, and back- 

 ward, underneath the brain. Here it forms a hollow, bilobed transverse band, 

 conspicuous in all subsequent stages, when the brain is viewed from the haemal 

 side, but almost entirely concealed below the hemispheres when seen from the 

 neural side. (Figs. 40-42, 43, 46, 47.) 



