RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 865 



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 that the main part of the 

 'mesoblast of the trunk is not directly derived from the primitive 

 streak. 



Before leaving this stage we would call attention to the pre- 

 sence, in one of our blastoderms of this stage, of a deep pit at 

 the junction of the primitive streak with the region in front of it 

 (PI. 44, Ser. F, I and 2). Such a pit is unusual, but we 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 consti- 

 tutes the head fold. 



PI. 45, 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, i). Below the 

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

 into two layers, but continuous laterally, both with the hypo- 

 blast 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 ap- 

 pearance. 



