x I -oWL TIRST DAY 521 



For a, short distance in the region of its from end the mass of 

 cells forming the primitive streak is continuous not only with the 

 .'(((..It-nil out with tin- endoderm as well: tin- primitive streak of 

 this region may In- delined as a tract along which there is cellular 

 coniiniiity lu-t ween the ecloderm and the endoderm. 



During the latter half of the first day what is known as the 

 Head process M makes its appearance, (Fig. 225, B, hji). In a view 

 of the whole blastoderm this has the appearance of being a somewhat 

 less distinct prolongation forwards of the primitive streak in front 

 of the knoh which marks its apparent front end. The study of 

 transverse sections shows that the so-called head process is exactly 

 similar in structure to the primitive streak immediately behind it, 

 except that it is separated from the overlying ectoderm by a distinct 

 split and that there are no primitive folds or primitive groove 

 over it. On its lower side there is perfect continuity with the 

 endoderni as is the case with the anterior part of the obvious 

 primitive streak into which it is continued. 



During the same period of incubation there appears the first 

 sign of the surface relief of the body of the embryo in the form 

 of what is known as the head fold (Fig. 227, A, h.f). This is 

 formed by the blastoderm bulging upwards and forwards, forming a 

 projection bounded in front by a steep face crescentic in shape as 

 seen from above, the two horns of the crescent directed backwards. 

 The projection increases in prominence : its front edge soon comes to 

 overhang, the blastoderm becoming tucked underneath it both in front 

 and at the sides, the two horns of the crescent which the fold formed 

 at its first appearance giadually extending farther and farther back- 

 wards. The projection is destined to give rise to the head end of the 

 embryo and there are certain important details to be noticed about 

 its structure which can be made out best by the study of sagittal 

 sections. 



The region of the blastoderm where the head fold develops is 

 composed of the two primary layers, ectoderm and endoderm, the 

 mesoderm not yet having spread into it. It follows that the head 

 rudiment has a double wall, its outer sheath of ectoderm enclos- 

 ing an inner wall, quite similar in shape, composed of endoderm. 

 It will be understood that this inner wall of endoderni is continued 

 at its hind end into the flattened layer of endoderm which lies on the 

 sui lace of the yolk. In other words the endoderm within the head 

 rudiment may be described as forming a very short wide tube, blind 

 anteriorly but opening behind into the yolk. This endodennal tube 

 is the rudiment of the front part of the endodermal lining of the 

 alimentary canal of the adult and is termed the foivgut. 



Soon after the commencement of the formation of the head fold 

 the ectoderm of the medullary plate becomes raised up into a longi- 

 tudinal ridge (Fig. L'27, A, m.f) upon each side of the median line. 

 Between the two ridges is a groove the medullary groove: the 

 ridges themselves are the medullary folds : the two medullary folds 



