No. 7. — The Segmentation of the Nervous System in Squalus 

 acanthias. A Contribution to the Morphology of the Verte- 

 hrate Head} By H. V. Neal. 



CONTENTS. 



Introduction I'iS 



Criteria of Segmentation 1-18 



Summary of liesults of former Inves- 

 tigations on tlie Segmentation of 



the Encepbialon 150 



I. Locy's "Neural Segments" or 



"Metameres" .... 154 



a. Material 154 



b. Method of Study . . . .155 



c. Description of Locy's '" Neu- 



ral Segments " .... 156 



d. Continuity of the " Seg- 



ments" 159 



8. Interpretation of the Evi- 



■ dence 161 



f. Limit of "Cephalic Plate" . 162 

 II. The " Hindbrain Neuromeres" in 



S. acanthias 166 



a. Definition of the Term " Neu- 



romere " 166 



b. Development of Hindbrain 



Neuromeres 167 



c. Summary 172 



III. The Neuromeres in the Trunk Re- 



gion 173 



a. Development of Myelo- 



meres 173 



b. Summary 176 



IV. The Neuromeres anterior to the 



Hindbrain 177 



a. Essential Criteria of Neuro- 



meres 177 



b. Development of the Fore- 



brain and Midbrain . . . 179 



c. Summary 18,5 



Y. The Relation of Neuromeres to 



Somites 186 



a. Relation of Myelomeres to 



Somites 186 



Page 



b. Relation of Encephalomeres 



to Somites 187 



c. Somatic Value of the Pre-otic 



Mesoderm Segments . . 187 



d. Summary 206 



VI. The Relation of Neuromeres to 



Nerves 207 



a. Historical Review . . . 208 



b. Nerve Relations in the 



Trunk of S. acanthias . . 210 



c. Nerve Relations in the 



Cephalic Region .... 211 



d. Development of the 



1. Oculomotorius . . . 220 



2. Abducens 230 



3. Trochlearis .... 235 

 VII. Segmental Value of Hindbrain 



Neuromeres 240 



a. Non-phylogenetic Interpre- 



tation 240 



b. Phylogenetic Interpretation 243 



c. Interpretation of Hindbrain 



Neuromeres in S. acan- 

 thias 245 



VIII. Primitive Relations of Cephalic 



Segments I.-VH. . . . 254 

 254 

 255 

 255 

 256 

 258 

 258 

 259 



a. Encephalomere VII. 



b. Encephalomere VI. 



c. Encephalomere V. 



d. Encephalomere lY. 



e. Encephalomere III. 



f. Encephalomere II. 



g. Encephalomere I. . 

 h. Comparison with the Seg- 

 mentation of Amphioxus . 260 



i. General Conclusions . . . 268 



IX. Summary 27,5 



Bibliography 279 



Description of Plates 293 



1 Contributions from the Zoological Laboratory of tlie Museum of Comparative 

 Zoology at Harvard College, under the direction of E. L. Mark, No. LXXXIX. 



VOL. XXXI. — NO. 7. 1 



