YOUNG TWIN HUMAN EMBRYOS WITH 17-10 PAIRED SOMITES. 37 



show little increase in length or width of the cups, but there is a remarkable increase in 

 depth and a very much more constricted opening to the exterior, showing how rapidly the 

 cup is becoming converted into a vesicle. 



THE NERVOUS SYSTEM. 



The nervous system is in the stage where closure has just occurred and is complete, 

 except for the anterior and posterior neuropores. The fusion of the lips of the neural 

 groove is not very heavy and over most of the hindbrain and again over a large extent of 

 the yolk sac the seam has opened in both of the embryos studied. Careful study of the 

 sections shows, however, that there had been fusion, ragged edges being evidence of this, 

 as is also the fact that the ectoderm is broken, and not continuous with the walls of the 

 neural canal, as it would be if fusion had not yet occurred. 



The anterior neuropore (plate 1 and plate 3, fig. 5) is not situated exactly at the most 

 anterior point of the head, but on the surface just ventral to this, so that it looks vent rally 

 as well as forward. It is situated in a shallow depression and is very wide open and looks 

 directly into the cavities of the optic vesicles as well as into the lumen of the rest of the 

 forebrain. 



The posterior neuropore represents the still unformed, or at least undeveloped, portion 

 of the spinal cord, and is wide open. It will be described later. 



It will be seen thus that the nervous system is still open both in front and behind at 

 the neuropores, but closed everywhere else. In Low's (1908) embryo, Pfannenstiel III, of 

 14 somites, a large part of the nervous system is still open in front, so that quite an advance 

 in closure is shown by the embryos here described. Van den Broek's (1911) embryo of 22 

 somites and Thompson's (1907) of 23 somites show complete closure of the anterior neuro- 

 pore. Embryo V and VI thus show a degree of closure which is coincident with their 

 stage of development. 



The nervous system conforms to the general contour of the dorsal surface of the embryo 

 and exhibits one important flexure, the cephalic. The anterior part of the brain is flexed 

 to form exactly a right angle with the rest, the flexure occurring in the region of the mid- 

 brain. In general contour and appearance the brain shows a remarkable correspondence 

 to figs. 26 and 27 in the account of the nervous system in Keibel and Mall's Embryology. 

 To make their figure serve here it would be necessary only to make a wider neuropore and 

 indicate the neuromeres. 



The anterior end of the nervous system, forming the brain, is divided distinctly into 

 the three primary vesicles, each of which in turn shows a series of secondary divisions, 

 forming the anlagen of many future structures in the brain (plate 3, fig. 5). These anlagen 

 are in the form of the total folds, as described by Broman (1896) and Mrs. Gage (1905). 

 By a total fold I mean one involving the whole brain wall and indicated by a bulging of 

 the exterior coincident with an enlargement of the lumen, and bounded by furrows exter- 

 nally, which form corresponding ridges on the interior of the wall. 



The forebrain (plate 3, fig. 5) as a whole is narrow from side to side and very deep in 

 its dorsoventral diameter. Its lumen is narrow and slit-like. There are five distinct 

 regions delimited in the forebrain, as follows : 



The first is the optic evagination. This involves nearly the whole dorsoventral extent 

 of the lateral wall of the brain just behind the anterior neuropore. There is yet no differen- 

 tiation into optic stalk and optic cup, and the whole evagination is flattened so that its 

 lumen is a vertical cleft, which opens in its whole dorsoventral extent into the lumen of 



