62 WILLIAM PATTEN. 



about the middle of the hsemal surface of the brain, 

 enclosed by a common envelope for the brain and optic 

 ganglia (fig. 49). I know of no other Arthropod in which 

 the optic ganglia have this position. They usually project 

 from the sides, or, as in most Arachnids, from the neural 

 surface of the brain. In their position on the hsemal surface 

 the optic ganglia of Limulus resemble the thalamencephalon 

 of Vertebrates, with which I regard them homologous. The 

 other important changes that take place are in the cerebral 

 hemispheres. They arise from disc-like thickenings 

 of the cephalic lobes, and correspond in position with 

 the brain-lobes of the second segment of Scorpions 

 (compare figs. 24, 59, and 61, c. /«.). They grow upward at first, 

 and then their summits spread out mushroom-like, till they 

 completely conceal the remainder of the cephalic lobes, in- 

 cluding even the greater part of the segment of the mid- 

 brain (compare figs. 24, 25, 43, 47 — 52). 



By this method of growth a series of chambers are formed, 

 some disappearing early, others persisting in the adult, that 

 correspond to certain cavities of the Vertebrate brain, such as 

 the fifth ventricle, the lateral ventricle, the primary cerebral 

 vesicle, and the cavities leading from the third ventricle into 

 the lateral eye-tubes. 



We shall now describe these changes in more detail. I 

 have carefully studied these stages, both in surface views and 

 in sections, and have constructed a partial wax-plate model of 

 an embryo in about this stage. In fig. 24 I have represented 

 the cephalic lobes as they would appear in surface views. 

 Some details have been omitted, and the clearness of some 

 parts has been exaggerated, and in order to save repetition of 

 figures^ strict attention was not paid to synchronism. It is 

 therefore not so much an accurate picture of one stage, as a 

 diagram of two or three closely joined stages. A series of 

 sections of this period, drawn as accurately as possible, will 

 serve to make the meaning of the drawing clearer, and will 

 also show to what extent it is diagrammatic. Such a series of 

 longitudinal sections is shown in figs. 35 — 40. They are taken 



