14 DR. A. MILNES MARSHALL. 



points that I have succeeded in bringing to light, and one to 

 which I shall have occasion to refer subsequently. 



Figs. 6 to 10, representing transverse sections through the 

 head of a twenty-nine hours' chick, illustrate my next stage. 



The section from which fig. 6 is drawn passes through the ante- 

 rior part of the optic vesicles. It has a superficial resemblance to 

 tig. 3, but is taken much further forward than this latter, and should 

 rather be compared with fig. 2. The section, like that drawn in 

 fig. 2, consists entirely of epiblast ; there is no mesoblast or hypo- 

 blast in it at all. On the dorsal surface the medullary folds have 

 met, but not coalesced ; on the ventral surface coalescence has 

 occurred ; but there are still indications, in the shape of two slight 

 prominences of the external epiblast, of the stage preceding coa- 

 lescence, represented by fig. 2. 



The neural ridge forms a conspicuous outgrowth {m) on either 

 side of the summit ; its characters and relations are precisely the 

 same as those it possessed further back in the brain at an earlier 

 period. 



Fig. 7 passes through the constriction {fm b) separating the 

 fore brain from the mid brain ; it passes also through the fore 

 gut {/ g), the notochord [u), and through a small cellular rod 

 underlying the notochord, and lying between this latter and the 

 hypoblastic roof of the fore gut ; this appears to be the same 

 structure as that described by Balfour and Gotte as the " sub- 

 notochordal rod." The medullary folds have completely coa- 

 lesced, and the neural ridge (/«.) has attained a great size, forming 

 a very prominent feature in the section. 



Fig. 8 is taken through the widest part of the mid brain 

 (m b). The neural ridge [m) is very prominent, and slightly 

 larger than in the preceding figure. Though the neural ridge is 

 in contact peripherally with the mesoblast, the outlines of the 

 ridge are clear and well defined, and there is at this stage very little 

 difficulty in distinguishing between the small spherical, rather 

 closely compacted cells composing the neural ridge, and the 

 larger, irregular, branching, and loosely aggregated mesoblast 



cells. 



Fig. 9 passes through the middle of the hind brain and through 

 the hinder part of the fore gut {fg). The medullary folds 

 have not yet coalesced, but the neural ridge [m) is very con- 

 spicuous on either side. 



Fig. 10 is taken from the same embryo, a short way posteriorly 

 to fig. 9, and still in the hind brain. The external ei)iblast is 

 seen to be thickened laterally, forming the commencement of the 

 auditory epithelium [atcd). The section passes through the 

 anterior part of the mid gut [mg) ; the neural canal is as yet 

 unclosed, and the neural ridge {m) is very small. 



