340 JOHN WARREN 



Johnson (52) discusses the question of neuromeres agreeing 

 with the results of Hill and Locy and (53) he considers at length 

 the morphology of the fore brain. He lays great stress on the 

 shifting of the optic stalk from the primitive optic groove or 

 infundibular recess to the preoptic recess. He assigns the optic 

 vesicles to the diencephalon and states that the boundary between 

 diencephalon and telencephalon is the velum and a ridge extend- 

 ing from the velum to the floor of the brain and passing in front 

 of the neuromere to which the optic vesicle belongs. 



Johnson's (53) fig. 42 shows parasagittal sections of the brain 

 of a 7 mm. pig, fig. 34, a model of the brain of a 5 mm., and fig. 

 35 parasagittal sections of the brain of a 6 mm. pig. He finds 

 three neuromeres in the fore brain and two in the mid brain. The 

 first segment is the telencephalon, and the second is the optic ves- 

 icle, and the third lies behind the vesicle and supports later the 

 paraphysis. I am unable to observe in my models this shifting of 

 the optic vesicle and from the earliest stage I have studied it 

 seems always to be in front of the point where the optic chiasma 

 appears. Furthermore, the ridge which continues the velum is 

 seen in figs. 2 and 17 appearing thus very early and passes clearly 

 behind the optic stalk to end in the brain floor at the chiasma. 

 This relation is also shown in Johnson's fig. 34, the opening of the 

 optic stalk apparently lying in front of this ridge. I should 

 therefore include the optic vesicles in the telencephalon and count 

 his first two neuromeres as one segment. The second segment 

 would lie behind this ridge. In a pig of 7.5 mm. one can find a 

 trace of a third subdivision of the fore brain ; but this is seen much 

 better at a more advanced stage. Fig. 34, a pig of 10 mm. shows 

 a parasagittal section with three distinct segments in the fore 

 brain and two in the mid brain. This third segment is the syn- 

 encephalon, while the second is the parencephalon and produces 

 the epiphysis. Fig. 35 is a horizontal section of the same stage 

 and clearly shows these three subdivisions. They are also shown, 

 I think, in Johnson's fig. 41 (central section) of a 15 mm. pig. 

 The first segment in this figure is the telencephalon. Then 

 comes a rather long first diencephalic segment followed by a short 

 second diencephalic segment in front of the mid brain. This 



