504 'Journal of Comparative Neurology and Psychology. 



(1905) is a trifle larger and considerably more advanced in develop- 

 ment. In it the optic vesicles are connected with the pit formed in 

 the neuropore-space. This pit seems to correspond to the preoptic 

 recess above described. From these three embryos it appears very 

 probable that the course of development in man is the same as in the 

 pig and lower foinns. 



II. Discussion. 



1, The Anterior End of the Head and Brain. — Owing chiefly to 

 the lack of certain essential facts, an extensive literature has grown 

 up about the question of the anterior end of the brain. Since the 

 facts which were wanting are supplied in the preceding pages, a 

 detailed review of the discussions from v. Kupffer and His onward 

 would be unprofitable. The determination of the anterior end of 

 the brain is a matter of direct observation. In the study of succes- 

 sive stages in the early development of a selachian, an amphibian 

 and a mammal the essential facts are found to be perfectly clear, 

 and the three forms agree in the form changes of the brain and in 

 the relations of the brain to the ectoderm, entoderm and mesoderm. 



The anterior boundary of the neural plate is formed by a trans- 

 verse ridge, the terminal ridge, which is continuous with the neural 

 folds bounding the neural plate laterally. This terminal ridge is 

 clearly seen in successive stages and is readily followed up to the 

 time when the optic chiasma is formed in it. The optic chiasma 

 therefore occupies the anterior border of the floor plate of the brain. 

 This is a matter of fact, not of interpretation. 



Behind the terminal ridge lies a transverse groove which laterally 

 becomes continuous with the optic pits or vesicles. This is to be 

 seen from the earliest stages when the optic pits are recognizable in 

 the neural tube or, indeed, on the open neural plate. This groove 

 is the depression which His (1892, 1893) called the recessus infun- 

 dibuli and which v. Kupffer (1893) called the sinus postopticus. 

 Later workers have followed His, although his own numerous figures 

 (1892) show that there are two distinct depressions in the region 

 between the chiasma and the mammillary recess. It is clear also 

 (His, 1892, Fig. 2 and p. 162) that the opening into the saccus 



