220 CHESTER H. HEUSER 



the thalamus has enlarged, and the corpus striatum appears as 

 if pushed forward before it. Between the corpus striatum and 

 the upper part of the thalamus, the interventricular foramen is 

 seen as a slit, which arches up over the corpus striatum, being 

 widest immediately under the velum transversum. The area of 

 the aperture has become further reduced. Between the corpus 

 striatum and hypothalamus, the optic recess appears as in the 

 17-mm. specimen. Somewhat further back, and parallel with it, 

 is the recessus infundibuli, and between the two grooves made by 

 these recesses, is the pars optica hypothalami. This is convex 

 toward the ventricle. Between the hypothalamus below and the 

 thalamus above is the sulcus hypothalamicus (B.N.A.) or sulcus 

 limitans (His '93) — a groove which continues from the mid-brain 

 downward and forward becoming somewhat indistinct above the 

 pars optica hypothalami. Reichert ('61) described this groove in 

 the brain of a pig embryo (also in cat and human embryos) as 

 extending from the foramen of Monro to the entrance of the aque- 

 duct of Sylvius, and he named it the sulcus of Monro. In fig. 

 12 an extension may be traced from the sulcus to the lower end 

 of the interventricular foramen; another extension proceeds to- 

 ward the recessus postopticus. But the main continuation is 

 probably that which ends in the optic recess. This accords with 

 the opinion of His ('93, p. 177), and also of Johnston ('09, p. 517). 

 The sulcus limitans is indicated in the embryo of 17 mm. and in 

 younger specimens, but it is more distinct at 22 mm. 



The median section of the 22-mm. embryo shows the uncut 

 median surface of the right hemisphere, ending below in a rounded 

 olfactory lobe. Extending from the lower border of this lobe 

 toward the notch below the velum transversum, is a groove, 

 which, with the lamina terminalis, bounds a triangular portion of 

 the hemisphere. Smith ('03) named this triangular thickening 

 the ' paraterminal body'. Herrick ('10) proposes to divide this 

 body into a ventral component, the 'corpus precommissural,' 

 and a dorsal component, the 'primordium hippocampi,' but these 

 subdivisions do not appear externally in the 22-mm. embryo. 



In the dissection of a 45-mm. embryo (fig. 14) most of the para- 

 terminal body together with the adjacent medial wall of the right 



