Johnston, Forchrain Vesicle in Vcrtchrotes. 479 



Developiiiciit of the Forebrain. 



a. The Optic Vesicle and the Primitive Optic Groove. — The early 

 appearance of the optic vesicles on the open neural plate has been 

 described by Locy (1895), and the writer has elsewhere (1905, 

 1906, 1909) brought together the evidence that the optic vesicle is 

 derived from the alar plate of the embryonic neural tube. The 

 study of its relations for the present purpose begins with embryos 

 of about 15 somites. The medial surface of the right half of a 

 model of the head of such an embryo is shown in Fig. 5. The neural 

 tube has a cephalic flexure and is open at the neuropore and for a 

 short distance along the dorsal surface. The lateral wall presents 

 two concavities, a broad shallow one for the mid-brain and a deeper 

 one for the forebrain. The depth of this concavity is due chiefly 

 to the formation of the optic vesicle, which seems to involve the 

 greater part of the lateral wall of the forebrain vesicle. In the 

 floor of the brain the optic vesicles of the two sides are connected 

 with one another by a transverse groove or depression which has 

 heretofore been called the "infundibulum." That it does not be- 

 come the infundibulum will be shown farther on. Its relation to 

 the optic vesicles and its later history suggest that it be called the 

 primitive optic groove. Its relations to the preoral entoderm as 

 above described and to the neuropore as seen in Figs. 3, 4 and 5 

 show that this groove is in the floor-plate of the brain some distance 

 behind the anterior end of the entoderm. 



In the embryos with 17 somites. Figs. 3 and 4, the neuropore is 

 closed except at a single point which shows as a thin place in one 

 section of 6^/3 microns thickness. The distance from the primitive 

 optic groove to this point is greater than the distance to the lower 

 border of the neuropore in the embryo of 15 somites. The arrange- 

 ment of the cells in the lower lip of the closing neuropore as seen 

 in transverse and frontal sections shows that there is a process of 

 fusion of the lips of the neuropore from below upward and therefore 

 the last point of the neuropore to remain open is a point in the 

 dorsal seam of the neural tube some distance removed from the 

 anterior border of the neural plate. This part of the seam of closure 

 which represents the length of the neuropore is what is called the 



