DEVELOPMENT OF FOURTH DAY 



511 



medial processes grow toward the mouth and meet the maxillary 

 processes which grow inward from each side. It is the fusion of these 

 two naso-medial processes with each other in the midline and with the 

 maxillary processes laterally that forms the upper jaw, the maxilla. 



The lower jaw is formed by the fusion in the midline of the right 

 and left portions of the mandibular arch. 



Foramen of Monro 

 Corpus striatum 



Eye 



III ventricle 

 Ohonoid fissure 



Me'sodermal tissue, 

 forming later the 

 chorioid plexus 



Pharynx 

 Tongue 



Transverse section through the forebrain of a 16 mm. human 

 embryo (six to seven weeks) to show the relationship of the 

 ventricles. 



THE NERVOUS SYSTEM 



Figures 282 and 288 show how the two lateral evaginations of the 

 fore-brain stand in relation to the cephalic end of the central nervous 

 system, and why it is that the ears come to lie on practically the same 

 dorso-ventral level with the eyes, although they begin forming so far 

 ipart. 



The development which brings this about has already been dis- 

 ;ussed. Here it is important for the student to observe that the two 

 evaginations forming the telencephalic vesicles have an open space 

 within them, known as the I and II ventricles, also called lateral ven- 

 tricles. The portion between them is the III ventricle, which is later to 

 become a mere connecting slit-like tube to connect the lateral and more 

 posterior ventricles. The entire opening in the fore-brain is called the 

 telocoele, that in the diencephalon the diocoele, that of the mesencepha- 

 lon the mesocoele (later called the aqueduct of Sylvius), that of the 

 metencephalon the metacoele, and that in the myelencephalon the 

 myelocoele. 



Figure 282 also shows that what was once the most anterior part 

 of the fore-brain, i. e., the lamina terminalis, is no longer so, the lateral 

 vesicles having extended further forward. The telencephalic vesicles be- 

 come the cerebral hemispheres in the adult. These become so large that 

 they cover the entire diencephalon and mesencephalon. 



