Neal. — Nervous Syst. Squalus. 



PLATE 5. 



All the Figures except 31 and 32 represent frontal sections of embryos viewed 

 from the dorsal side. All except Figures 25, 32, and 35 are magnified 43 diameters. 



Fig. 22. A frontal section of an embryo with 14 or 15 somites. Encephalomere YV 

 appears as a thickening of the lateral walls of the neural tube. No 

 local thickening seen in the region of encephalomere III. 



Fig. 23. From an embryo with 16 or 17 somites. A local thickening of the lateral 

 walls in the posterior part of encephalomere III appears. 



Fig. 24. From an embryo with 19 or 20 somites. The first four hindbrain neuro- 

 meres are now seen as local tliickenings of the lateral walls, the 

 thickening of neuromere III affecting its posterior part only. 



Fig. 25. From an embryo with 28 to 30 somites, magnified 75 diameters. Five 

 hindbrain neuromeres are seen. The auditory invagination appears 

 opposite encephalomere VI. 



Fig. 26. From an embryo with 50 somites (8 mm.) in the region of the "Deck- 

 platte," showing the faintly marked expansions of the encephalomeres. 



Fig. 27. A more ventral section of the same embryo. The enceplialomeres sharply 

 defined by constrictions. A secondary constriction in encephalomere 

 III appears. 



Fig. 28. A more ventral section of the same series, in the region of the lateral 

 zones. The local thickenings of tlie encephalomeres well marked. 



Fig. 29. A still more ventral section of the same embryo. The inner cusps be- 

 tween the neuromeres appear as in the more dorsal sections (Fig. 27). 



Fig. 30. Frontal section in the region of the lateral zones of an embryo of 15 mm. 

 The structure of the neuromeres is seen to be the same as that 

 described by Orr ('87) for the Lizard. 



Fig. 31. Cross section of an embryo with 20 somites, in the region of encephalo- 

 mere IV, to show the thickening of the lateral zones. 



Fig. 32. Cross section of an embryo with 28 to 30 somites in the posterior region 

 of encephalomere III. The lateral zones more markedly thickened 

 than in the previous stage (Fig. 31). 



Fig. 33. Frontal section of an embryo with 50 somites (8 mm.), killed with a mix- 

 ture of chromic, picric, and sulphuric acids, showing great intensifica- 

 tion of the neuromeres, as the result of contraction due to inadequate 

 fixation. The drawing, however, exaggerates tlie phenomena, since it 

 represents the nuclear regions of the medullary wall with deeper 

 shading. 



Fig. 34. Frontal section of a 19-day Sivhie embryo. The constrictions between 

 the neuromeres are sharply defined. 



Fig. 35. Frontal section of an embryo of Amhlysfoma shortly after the closure of 

 the neural tube. The neural tube is sharply outpocketed in the regions 

 of proliferation of the ganglionic Anlagen of nerves V and VII. No 

 evidence of a tliickening or outpocketing comparable with encepha- 

 lomere IV appears either at this or later stages. 



