42 
MONICA TAYLOK. 
(d) Nervous System. 
The Brain. 
The course of events in the conversion of the flat, wide- 
spreading medullary plate into the solid keel-like neural 
rudiment, and the formation, out of the anterior part of this 
plate, of a brain-rudiment with ventricles which develop 
secondarily, is quite Teleostean in character. Except for a 
slighter wider diameter and the presence of optic vesicles 
there is nothing to distinguish the brain from the spinaTcord 
in the stages up to 20. In a dorsal view of Stage 22, however, 
the three ventricles are discernible, while sagittal sections of 
about this age (Text-fig. 4 a) show that the floor of the 
archencephalon is becoming delimited posteriorly by an 
uprising of the brain floor. 
Cranial flexure has increased considerably by Stage 24 
(Text-fig. 4 b), when the ventricles are very well defined, 
'bhe mesencephalon and rhombencephalon are conspicuous 
objects in a dorsal view of the whole embryo (PI. 1, fig. 7), a 
somewhat narrow portion connecting the latter with the 
former. Indications of a cerebellum rudiment are discernible, 
as two thickenings on the anterior wall of the rhombence- 
phalon cavity. In the prosencephalon a pineal rudiment is 
distinguishable. The optic recess is somewhat narrow. 
Cranial flexure continues to increase (Text-fig. 4 c), reach- 
ing a maximum in Stage 28 (Text-fig. 4 d). By this time 
(Stage 28) nerve-fibres have appeared. The posterior com- 
missui-e and habenular ganglia as well as anterior and post- 
optical commissures are formed. The paired thickenings 
alluded to above have grown backwards and inwards to form 
a rudimentary cerebellum, while on either side of the lamina 
terminalis the basal thickenings are converting the prosence- 
phalon into a solid structure with a thin non-nervous roof. 
By Stage 30 (Text-fig. 4 e) the brain has begun to straighten 
out and to assume much of its adult appearance in transverse 
sections. The rudiment of the velum transversum is present 
