218 CHESTER H. HEUSER 



or parencephalon, and the second or synencephalon of Kupffer 

 ('03). The relatively large cavities of the optic vesicles extend 

 out from the antero-ventral part of the diencephalon. Almost 

 one-fourth of the length of each vesicle, representing the optic 

 stalk, extends nearly straight out laterally, and the remainder 

 bends sharply toward the mid-brain. 



In an embryo of 12 mm. the lateral ventricles have expanded 

 considerably and they now slope outward forward and slightly 

 upward from the interventricular foramen. The foramen is 

 almost circular in outline and has an area of about 0.428 sq. mm. 

 In the cast (fig. 5) the rounded mass of wax filling the lateral 

 ventricle shows a slight concavity below, which has been pro- 

 duced by the developing corpus striatum. This body arises as a 

 thickening of the ventro-lateral wall of the hemisphere, as seen 

 in the dissection, figure 6. This figure, it may be noted, bears 

 a very striking resemblance to the reconstruction of the brain of 

 Anguis fragilis (40-50 somites), published by Kupffer ('03, p. 

 220, fig. 224). In both, the parencephalon and synencephalon 

 are sharply marked off from each other. Two swellings, which 

 correspond with these subdivisions, are seen in the cast (fig. 5). 

 The parencephalon is larger than the synencephalon, and its cavity 

 produces a more extensive swelling, which however, is quite low. 



The mamillary recess and the infundibulum below it, neither 

 of which had appeared in the 5.1-mm. embryo, are now very 

 distinct. The velum transversum, a portion of which is shown 

 in figure 7, is well developed, producing a deep inward bend of 

 the anterior wall of the fore-brain above the hemisphere. 



The brain of a 17-mm. pig embryo, as seen in median section 

 (fig. 8), shows a prominent corpus striatum, above which is the 

 interventricular foramen. The foramen is now crescentric, since it 

 has been invaded from below by the corpus striatum, but not- 

 withstanding the growth of the entire embryo, its area has become 

 actually reduced. It measures about 0.315 sq. mm. As seen in 

 the dissection, the lower portion of the corpus striatum is bounded 

 behind by a deep groove which is continuous with the optic recess. 

 This groove appears as a ridge upon the cast (fig. 9), and in front 

 of it the position of the corpus striatum is indicated by an excava- 



