NERVOUS SYSTEM OF SCORPION. 47 



invagination is forming in the median furrow. The tissue thus 

 invaginated is destined to form nerve tissue connecting the two 

 halves of the neuromere in each of the segments mentioned. The 

 process is carried backward to the posterior segments in later 

 stages (Figs. 4, 5). These invaginations have the same appear- 

 ance as the numerous pits so characteristic of the nerve tissue, 

 differing from the latter however in time of appearance and in 

 shape. The elongated shape is probably due to the narrowness 

 of the median furrow. More pits are constantly being added to 

 the neuromeres, and the late appearance of those in the median 

 furrow is more apparent than real. In no case have I observed 

 the invaginations of the median furrow to coalesce and form a 

 temporary central canal, as Patten claims to have seen. The 

 walls of the first pair of invaginations in the first neuromere have 

 thickened and are fusing in the mid-line to form the semi-lunar- 

 lobe of Patten (^/). The ectoderm has begun to grow back- 

 ward over the first neuromere, its free border being shown at cc. 

 On each side there is an invagination in the ectoderm close to 

 the free border, forming the median-eye-sac (oni), and an ecto- 

 dermal thickening, to form the lateral eyes (<?/). The ectoderm 

 has begun to grow over the ventral chain from the sides, but this 

 is not shown in the figure. Patten describes three pairs of gang- 

 lionic invaginations in the "cephalic lobes" the first of which I 

 have represented in Fig. 2, ci l , and Fig. 3, si. The second, he 

 describes in relation to the median eyes. I was unable to find it, 

 either in sections or surface views. It is most probable that he 

 referred to the ectodermal invagination or median eye sac (Fig. 

 3, oiii}. The third he describes in relation to the lateral eyes and 

 I have represented it in Fig. 2, ci~ ; it may be seen also in Fig. 

 3 near the lateral eye rudiment (<?/). 



Stage -/-.-- (Fig. 4.) A glance at Fig. 4 will show that a 

 piece is being constricted off of the posterior portion of each 

 ganglion in segments eleven to fourteen and it will be seen in 

 Fig. 5 that when the neuromeres begin to separate they do so at 

 this newly formed constriction. In other words the posterior 

 portion of one neuromere is constricted off and added to the 

 succeeding neuromere. Patten supposed the neuromeres to be 

 double, and used this fact, of which he was the first observer, to 



