113 



n^nov. 



->-(, olf.l. 

 poc.olf. 



Fig. 26. Frontal view of the brain of an embryo of 

 Squalus acantliias 25 mm long. X about 25 diameters. 



A frontal view of a Squalus embryo that has reached the length 

 of 25 mm is shown in Fig. 26. At this stage the new nerve is well 

 established, as is also the main olfactory. To obtain this view, the 

 front surface of the brain was completely exposed by first removing 

 the overlying layer of ectoderm, and then sweeping away the meso- 

 derm by the use of 

 an artist's brush, '^ 



and when necessary, 

 a needle. The ol- 

 factory cups have 

 been left in position. 

 The mark left by 

 the closing of the 

 neuropore is seen in 

 the median plane, 

 and on each side of 

 it the new nerve 

 (w. wov.), with a gan- 

 glionic enlargement 

 (gfn), which is con- 

 nected by two slender 



roots vnth the brain. From this position the nerve extends laterally 

 across the front surface of the brain and joins the main olfactory. 

 It is to be noted that the latter consists of two divisions (w. olf. l, 

 n. olf. m.), as described for adults, each of which is composed of 

 smaller bundles. The new nerve is mainly connected with the lateral 

 division (n. olf. I.). 



A magnified section of the brain of an embryo 20 mm long is 

 represented in Fig. 3. The section lies in the plane of the new nerve 

 and shows its point of attachment to the brain, the ganglion, and the 

 course of the nerve, from near the neuropore to the olfactory. 



The new nerve, on the right half of the brain of an embryo 38 mm 

 long, is shown in Fig. 27. Part of the olfactory cup has been broken 

 away, exposing the olfactory membrane, which is already thrown into 

 folds. The new nerve {n. nov.) is relatively long and slender, its 

 ganglion (gn.) is located near the closed neuropore and is connected 

 to the brain by two slender roots. The olfactory lobe is just begin- 

 ning at this stage but the bundles of the olfactory, already long 

 established, are large and more complex than in the previous figure. 



The new nerve as it approaches the main olfactory, becomes 

 flattened and therefore broader. It passes in front of the median 



Anat. Anz. XXVI. Aufsätze. 8 



