No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 393 



10. The Neural Folds in Amblystoma. 



The earliest stage in which the foundations of the neural 

 folds are visible is represented in PI. XX, Fig. 4. A trans- 

 verse section shows slight lateral thickenings of the epiblast 

 which, in a few hours, give rise to the bands («./.) shown in 

 Fig. 5 ; these bands arise independently, and, so far as I am 

 able to determine, differentiate in situ. They are later united 

 anteriorly by a transverse thickening which forms the cephalic 

 portion of the continuous fold. Clarke ('80) describes the folds 

 as extending forward until they meet at the opposite end of 

 the ^g^. Transverse markings such as shown in PI. XXII, 

 Fig. 9 (Necturus), are occasionally present in the neural plate 

 at this time; to these, however, I give little emphasis, since I 

 believe in most cases they are foldings between the myomeres, 

 which at this time are beginning to form. 



PI. XX, Fig. 6, and PI. XXII, Fig. i, represent respectively 

 the posterior and anterior portions of the same ^%% a few hours 

 later. The neural bands have united to form a continuous 

 fold, the cephalic portion having become especially prominent ; 

 the neural groove has appeared and extends to a point just 

 within the cephalic fold. At the anterior end of the neural 

 plate, on either side of the neural groove, is a slight depres- 

 sion, which in general outline is more or less circular, and in 

 many cases pigmented ; although by no means constant, their 

 occasional presence is suggestive, since, as we shall see later 

 in other forms, these areas form the bases of the paired eyes. 

 A section of such an embryo is shown in PI. XXII, Fig. 17: 

 the foundation of the nervous system is a broad, thickened 

 plate of epiblast, consisting of the epithelial {ep) and sensory 

 layers {s.ep) ; beyond the region of the folds («./.), the sensory 

 epiblast thins to a single layer of cells. Beneath the median 

 portion of the neural plate there is a single layer of hypoblast 

 {hy) which forms the basis of the chorda {ch). On either side 

 of the chorda the sheets of so-called gastral mesoblast {nies) 

 are well defined, although not yet split into somatic and 

 splanchnic layers. Just within the neural folds, on either side 

 of the median line, are the depressions (p.p) described above, 



