Neal. — Nervous Syst. Squalus. 



PLATE 4. 



Figs. 18 and 21 magnified 23 diameters. Figs. 19 and 20 magnified 21 diameters. 



Fig. 18. An embryo with 52 somites (8 mm.). The otic capsule now lies opposite 

 encephaiomere VII. The thalamic and trochlear portions of the tri- 

 geminus appear only as scattered clumps of cells. Posterior com- 

 missure clearly differentiated. Between this and the preceding stage 

 the oculomotorius has appeared as a fibrillar process from the ventral 

 wall of the midbrain, near gn. ms-ce. 



Fig. 19. An embryo with 65 somites (10 mm.). The chief peripheral branches of 

 the cranial nerves have appeared; the abducens, as a process from the 

 ventral wall of encephaiomere VIL 



Fig. 20. An embryo with 78 to 80 somites (16 or 17 mm.). In this stage the ramus 

 opthalmicus superficialis trigemini appears to have fibrillar relation 

 with the mesoderm of the 2d somite, which is growing forward. The 

 fibrous process of the abducens has come into relation with the 3d 

 somite, and also is seen to have a branch passing to the mesoderm pos- 

 terior to its place of origin. Most of the fibres of the ramus mandibu- 

 laris trigemini appear in connection with encephaiomere III. 



Fig. 21. An embryo 21 or 22 mm. long. The trochlearis is now differentiated, and 

 in relation with the muse, obliquus posterior. 



