166 THE GERM LAYER DERIVATIVES 



uniform staining reaction, and a rather static structural development. 

 The anterior lobe becomes highly vascular, consists of several types 

 of cells, and exhibits increasing complexity with further development. 

 The acidophils of the anterior pituitary gland become highly differ- 

 entiated and the basophils become poorly differentiated when the 

 thyroid gland is either poorly developed or inactive. Conversely, thy- 

 roid activity is correlated with hyperactivity of the basophils of the 

 anterior pituitary gland. This gland is formed from ectoderm, but 

 some is epithelial and some is brain ectoderm. Between the infundib- 

 ulum and the tuberculum posterius is a secondary and posteriorly 

 directed pocket known as the mammillary recess. 



The optic vesicles begin to develop very early as lateral outgrowths 

 from a slightly ventral level of the diocoel. The expansion of the 

 diocoel provides a temporary and slight thinning of the walls of the 

 optic vesicles. However, as these vesicles make contact with the lateral 

 head ectoderm, that portion of the vesicle in contact begins to thicken 

 and then invaginate to form a 2-layered optic cup. The most lateral 

 and invaginated portion of the cup will become the retina, the mesial 

 layer will become the pigmented layer of the eye, and the connecting 

 and somewhat constricted tube the optic stalk. The nervous elements 

 of this optic stalk will join in the optic chiasma which contains the optic 

 nerve fiber tracts from the two sides. The stalk will develop around 

 an inverted groove (the choroid fissure) which will contain, within 

 the groove, accessory nerves and blood vessels which feed the retina. 



The Mesencephalon (Midbrain). 



This portion of the brain functions largely as a pathway of nerve 

 tracts between the anterior prosencephalon and the posterior rhom- 

 bencephalon. These tracts are found principally within the paired 

 ventro-lateral thickenings of the walls and floor on either side of the 

 tuberculum posterius. They are known as the crura cerebri. 



The original dorsal thickening becomes subdivided by a median 

 fissure into paired dorso-lateral thickenings, at about the 1 mm. stage. 

 These are known as the optic lobes or corpora bigeniina. They do not 

 reach their full development until the time of metamorphosis. An- 

 terior to these lobes is the posterior commissure. From the posterior 

 limits of the mesencephalon and optic lobes may be seen the valvulae 

 cerebelli and the fourth pair of cranial nerves (trochlear) which 

 emerge from the dorso-lateral wall. The original cavity of the mid- 



