334 WILLIAM PATTEN. 



rior median opening. Owing to its mode of formation the sac 

 is at first very broad and bilobed^ but it is rapidly reduced to 

 the size of the future median eye. The lines a and b in 

 Fig. 5^ B, on the right, show the successive positions assumed 

 by the lateral limb of the ganglionic invagination. 



The outline of the cavity itself is only shown in this figure 

 in the last two stages; in the earlier stage the cavity is 

 shaded, in the later surrounded by a dotted line. At this 

 period the outer wall of the sac is formed by the closely 

 united and well-developed eyes (Fig. Q, c). It is evident 

 that this sac is in no sense an optic vesicle, nor, strictly 

 speaking, the cavity of a ganglionic invagination, although 

 derived from one. I shall call it the optico-ganglioni c 

 vesicle. 



Almost to the time of hatching it extends backwards a short 

 distance as a rather thin-walled tube, opening outward by a 

 narrow pore (Fig. 5, b, n. p., and Fig. 6, f). By this time 

 the optico-ganglionic vesicle is secondarily shut off from the 

 brain ; otherwise the pore would lead directly into the cerebral 

 cavity. 



The carrying of the originally lateral eyes toward the 

 median line also affects the optic ganglia to this segment, 

 folding them over the brain as shown in Fig. 5, b and d. 



The ganglionic invagination of the third segment is in all 

 respects like a typical ganglionic invagination in Acilius ; it is 

 deep and well defined, but does not involve the optic plate ; 

 hence the difference between the lateral and the median eyes 

 of Scorpio. It is important to notice that while all the 

 ganglionic invaginations of Scorpions are deeper and larger 

 than those in Acilius, in both cases they decrease in size and 

 depth from the first to the third. 



Although the entire anterior portion of the fore-brain of 

 Scorpio is practically invaginated, so that the brain and its 

 epithelium form the floor of a great complicated sac, the 

 brain itself takes no active part in the invagination. It is 

 enclosed solely by the extension of the lateral ganglionic in- 

 vaginations, and the consequent inward and backward growth 



