V.] THE MID-BKAIN AND HIND-BRAIN. 93 



completely surrounded by a ring of cartilage. The mesoblast-cells immediately 

 around it do not. however, exhibit any signs of becoming cartilaginous. 



By the tenth day the opening of the pituitary vesicle into the threat becomes 

 almost obliterated, and the lumen of the vesicle itself very much diminished. 

 The body itself consists of anastomosing cords of hyp 'blast-cells, the meso- 

 blast between which has already commenced to become vascular. The cords 

 or masses of hypoblast cells are surrounded by a delicate membrana propria, 

 and a few of them possess a small lumen. The infundibulum has increased in 

 length. 



On the twelfth day the communication between the pituitary vesicle and 

 the throat is entirely obliterated, but a solid cord of cells still connects the 

 two. The vessels of the pia mater of the vesicle of the third ventricle have 

 become connected with the pituitary body, and the infundibulum has grown 

 down along its posterior border. 



In the later stages, all connection is lost between the pituitary body and 

 the throat, and the former becomes connected with the elongated processus 

 infundibuli. 



Such is "Wilhelm Midler's account. Goette , however (Archir. Micr. Anat. 

 IX. p. 397), has recently given reasons for thinking that the pituitary diverti- 

 culum arises not from the closed foregut, lined v\ith hypoblast, but from the 

 buccal cavity lined with epiblast. He states that in its earlier stages it may be 

 seen to start on the oral side of the partition, which for some time divides the 

 secondarily formed buccal cavity from the primarily formed foregut, and 

 therefore, belonging to the former, cannot be reg-irded as the natural anterior 

 termination of the latter. 



Beyond an increase in size, which it shares with nearly 

 all parts of the embryo, and the change of position to which 

 we have already referred, the mid-brain undergoes no great 

 alteration during the third day. Jts roof will ultimately 

 become developed into the corpora bigemina or optic lobes 

 (quadrigemina in mammals), its floor will form the crura 

 cerebri, and its cavity will be reduced to the narrow canal 

 known as the iter a tertio ad quartum ventricidum. 



In the hind-brain, or third cerebral vesicle, that part 

 which lies nearest to the mid-brain, becomes during the 

 third day marked off from the rest by a slight constric- 

 tion. This distinction, which becomes much more evident 

 later on by a thickening of the Avails and roof of the front 

 portion, separates the hind-brain into the cerebellum in front, 

 and the medulla oblongata behind. AVhile the walls of the 

 cerebellar portion of the hind-brain become very much 

 thickened as well at the roof as at the floor and sides, the 

 roof of the posterior or medulla oblongata portion thins out 

 into a mere membrane, forming a delicate covering to the 

 cavity of the vesicle (Fig. 26. iv), which here becoming 

 broad and shallow with greatly thickened floor and sides, is 



