IIEN'KWED STUDY OF GERMINAL LAYERS OK THE CIllCK. 185 



considerable changes^ including a multiplication of its cells ; and 

 that these changes result in its becoming differentiated on each 

 side of the middle line, with more or less distinctness, into (1) a 

 hypoblastic sheet below, formed of a single row of flattened cells, 

 aiul (2) a mesoblast plate above formed of stellate cells, while in 

 the middle line there is a strip of stellate cells in which there is 

 no distinct differentiation into two layers. 



Since the region in which these changes take place is that in 

 which the medullary plate becomes subsequently formed, the 

 lateral parts of the mesoblast plate are clearly the permanent 

 lateral plates of the trunk, from which the mesoblastic somites, &c., 

 become subsequently formed ; so thai the main part of the meso- 

 blast (f the trunk is not directly derived frmn the primitive streak. 

 l^efore leaving this stage we would call attention to the presence, 

 in one of our blastoderms of this stage, of a deep pit at the junc- 

 tion of the primitive streak with the region in front of it (PI. XIV, 

 Ser. ¥, 1 and 2). Such a pit is unusual, but M^e think it may 

 be regarded as an exceptionally early commencement of that 

 most variable structure in the chick, the neurenteric canal. 



The next and last stage we have to deal with is that during 

 which the first trace of the notochord and of the medullary plate 

 make their appearance. 



In surface views this stage is marked by the appearance of a 

 faint dark line, extending forwards, from the front end of the 

 primitive streak, to a fold, which has in the mean time made its 

 appearance near the front end of the area pellucida, and con- 

 stitutes the head fold. 



PI. XV, Ser. K, represents a series of sections through a blas- 

 toderm of this stage, which have been selected to illustrate the 

 mode of formation of the notochord. 



In a section immediately behind the head fold the median part 

 of the epiblast is thicker than the lateral parts, forming the first 

 indication of a medullary plate (Ser. k, 1). I3elow the median 

 line of the epiblast is a small cord of cells, not divided into two 

 layers, but continuous laterally, both with the hypoblast and 

 mesoblast, which are still more distinctly separated than in 

 the previous stage. 



A section or so further back (Ser. k, 2) the axial cord, which 

 we need scarcely say is the rudiment of the notochord, is thicker, 

 and causes a slight projection in the epiblast above. It is, as 

 before, continuous laterally, both with the mesoblast and with 

 the hypoblast. The medullary plate is more distinct, and a 

 shallow but unmistakable medullary groove has made its 

 appearance. 



As we approacli the front end of the primitive streak the 

 notochord becomes (Ser. k, 3) very much more prominent. 



