361 
neural cavity is fully formed (embryos about 20 days old, 31 somites). 
Between 20- and 36-day stages I have a very complete series of divided 
embryos and have been able to trace in them unbroken continuity of 
the segments. Figs. 8, 9 and 10 represent three stages in this series. 
Op-n. inf 
Fig. 8. Salmo purpuratus. Right half of the anterior portion of a divided 
embryo with 31 somites, 20 days old. The internal surface of the encephalon is exposed 
to view. Letters and figures the same as in Fig. 7. >< 60 diameters. 
Fig. 9. Salmo purpuratus, Left half of the anterior portion of a divided 
embryo with 44 somites, 28 days old. Internal view of encephalon. Letters and figures 
the same as in Fig. 8. \ 60 diameters. 
In each case, the inner surface of the encephalon is exposed. It will 
suffice for the purpose of this paper to merely point out the im- 
portant features of the figures. 
The transverse groove (f) marks the anterior limit of the medulla 
and forms the dividing line between segments 6 and 7. The transvere 
