NEAL-. NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 183 



Two frontal sectious of an embryo at tliis stage are seen in Figures 51 

 and 52, Plate 7. Anteriorly in the more dorsal section (Fig. 51) is seen 

 the expansion of the prosenceplialou. Behind this lies an expansion 

 which might be considered as a nearomere, if a radial arrangement of 

 nuclei and a constriction of the brain wall were alone considered suffi- 

 cient criteria for such a structure. Since, however, it is simply a dorsal 

 expansion, which is unrelated to nerves, and soon becomes differentiated 

 into adult organs, I am unable to regard it as a neuromere. From it are 

 differentiated " Zirbelpolster " (parencephalou, Kebeuhirn, or Zwischen- 

 hirnblase) and epiphysis. Posterior to the constriction marked by 

 tlie arrow, which corresponds with the point so marked in Figure 50, 

 is situated a long expanded portion of the eucephalon which passes with- 

 out constriction into the midbrain vesicle. In the more ventral section 

 (Fig. 52), however, there is seen in this region a constriction which may 

 be traced ventrally to that point from which the anterior root of the 

 oculomotor arises. Two neuromere-like expansions, separated by tlie 

 constriction between primary forebrain and midbrain, are seen in this 

 stage as in the previous stage described. 



Passing now to a much later stage (21-22 mm.), we find (Plate 4, 

 Fig. 2 1) that the posterior commissure has come to lie much nearer the 

 base of the stalk of the epiphysis, and thus that the portion of the dor- 

 sal wall which is called by KupfFer diencephalon has become much re- 

 duced in the region of the midbrain vesicle. Thus it has come about 

 that frontal sections in a plane midway between the dorsal and the ven- 

 tral walls of the neural tube (Fig. 53, Plate 7) show only a single neuro- 

 mere-like expansion. In more dorsal as well as more ventral sections 

 this undei'goes constriction, so that it is by no means a simple neuro- 

 meric enlargement. A median sagittal section, such as that shown in 

 Figure 21 (Plate 4), is the most satisfactory for the study of segmenta- 

 tion at this stage. The primary forebrain is now differentiated into the 

 successive dorsal dilatations epencephalon, paraphysis (parencephalon), 

 and epiphysis. Dorsally the midbrain still continues to be a simple 

 expansion, while ventrally traces of the three segments still remain, 

 the anterior one having become much reduced in length. 



"With the exception of Locy, Zimmermann ('91) is the only investigator 

 who has studied the "neuromeres" in Selachii. For the purpose of 

 comparison, it is well to state his results hei'e. He finds at first eight 

 " primare Abschnitte " in the encephalon, the first three of which 

 exceed in size the last five. The first three are the Vorderhirn, Mittel- 

 hirn, and Hinterhirn, each of which he regards as a complex of en- 



