DEVELOPMENT OF CRANIAL NERVES IN THE CHICK. 11 



I have had to abandon as impracticable any attempt to distinguish 

 between them in my drawings. 



A great part of my work was done during the past summer 

 at the zoological laboratory of the University of Cambridge; 

 the remainder at St. Bartholomew's Hospital, London. 



I am indebted to Mr. Balfour for kind and valuable assistance 

 of many kinds received during the course of my investigations. 



The earliest stage in the development of the nerves is shown 

 by an embryo of nominally twenty-seven hours, but really cor- 

 responding to a typical twenty-two hours' chick.i In this 

 specimen the medullary folds have arched over towards one 

 another, and nearly met in the region of the head and neck, 

 but have nowhere coalesced completely ; while none of the 

 protovertebrse have yet been definitely estabhshed. 



Plate II, fig. 1, represents a transverse section through the 

 middle cerebral vesicle of this specimen. The medullary folds 

 have just met one another, but have not coalesced. The external 

 epiblast {ejj) is thin, and consists of but a single layer of cells ; 

 the wall of the cerebral vesicle itself [m, h) consists of some- 

 what elongated cells, about three deep, closely apposed to one 

 another, and placed with their long axes perpendicular to the 

 surface of the vesicle. Towards the summit of the vesicle these 

 cells become more spherical and rather less compactly ari'anged ; 

 and at the angle where the external epiblast turns in to form 

 the neural canal there is a small, but evident, outgrowth of these 

 spherical cells, forming on either side a projection that in trans- 

 verse section is somewhat conical in shape {m). 



This outgrowth {m) we shall find from its subsequent his-^ 

 tory constitutes the earliest stage in the development of the 

 nerves in the chick. On examining the sections in front of 

 and behind that figured, the outgrowth is found to form on 

 either side a longitudinal ridge, which is most prominent in the 

 section figured, and gets rapidly smaller in both directions, dis- 

 appearing completely at the constriction separating the middle 

 from the anterior cerebral vesicle, and extending very little 

 further in a posterior direction. This ridge I propose to speak 

 of in future as the neural ridge. 



A word of explanation is here necessary. Foster and Balfour 

 state^ that " for a brief period," after closure of the medullary 

 folds, " the calibre of this tube (neural canal) is uniform through- 

 out." This is not the case. Before the neural canal is closed 

 at any point in its length the anterior cerebral vesicle is a con- 



^ Foster aud Balfour, 'Elements of Embryology,' part i, p. 53. 

 ^ Op. cit., p. 58, 



