404 



CHARLES HILL, 



II. Descriptive. 



A. Encephalic Segments of Teleosts. 



a) Neural Segments of Salmo observed in Living and 

 Dissected Embryos. 

 I have found 11 encephalic segments in Salmo, and have traced 

 their history from embryos with 19 somites, 16 days old, up to 

 embryos with 35 somites, 38 days old. 



Fig. A. 



é : 1 ! 



Sketch A is a camera tracing of the anterior portion of a living 

 embryo of Salmo purpuratus, 16 1) days old. The optic vesicles are 

 present as two solid, elHptical, cellular masses that are turned caudad 

 and applied closely against the lateral walls of the encephalon (oi). ves). 

 The proliferation of the optic vesicles begins in embryos 2 days 

 younger. Sections of embryos of this age reveal the presence of the 

 lens, which is still attached to the epiblast. The central canal of the 

 neural axis has not yet appeared. Along the dorsal crest of the 

 cephalic region are 11 transverse grooves or constrictions that divide 

 the encephalon into 11 segments or joints (1 — 11). The transverse 

 groove (/■), that lies midway between the optic and the auditory 

 vesicles, is deeper than the other grooves and marks the posterior 

 border of segment 6, which later forms the cerebellum. It is de- 

 sirable to call particular attention to this groove, as in later stages it 

 becomes very conspicuous and forms clearly the dividing line between 

 the cerebellum and the medulla, enabling us thus to identify with 



1) See observations on age in relation to temperature of the water 

 p. 408. 



