Charles Sedgwick Minot 



85 



and immediately behind it has appeared the much larger posterior com- 

 missure, p. c, the position of which — compare also Figs. 5-10 — indi- 

 cates that it belongs rather to the mid-brain than to the fore-brain. 

 Opinion on this point must be reserved however until determined by an 

 investigation that will fix the boundary between the two primary cere- 

 bral vesicles. Our present information seems to me insufficient to de- 

 cide with finality this point. Both commissures are developed in the 

 " edoglia." This term I propose as the equivalent of the " Rand- 

 schleier " of His, to designate the outermost of the three primary layers 

 of the medullary wall.^ In stained sections the commissures are con-' 

 spicuous owing to the absence of nuclei, in contrast with the adjacent 



Fig 5. Embryo of 23 mm. Sagittal series, 231, section 79. x 30 diams. 



Fig. 6. Embryo of 28.0 mm. Sagittal series, 233, section 122. x 30 diams. A, 

 approximate plane of the section represented in Fig. 11 ; B, approximate plane of the 

 section represented in Fig 12. 



tissue of the brain-wall, which is densely crowded with nuclei. The 

 cross sections of the transverse nerve fibers are sharply marked in the 

 sections. 



Embryos of 22 mm. — 4 mm. longer than the last — show a decided 

 growth of all the parts, and the growth is downward, i. e. towards the 

 interior of the brain. The ectoderm, Ec, is still a thin layer, and the 

 mesoderm between it and the brain-wall has increased a little in thick- 



* Upon the fundamental morphological importance of these three layers I have in- 

 sisted in my Human Embryology, p. 616. The entire structure of the adult brain and 

 cord should be stated in terms of these three layers. 



