IA SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



thickened and may be called the primitive streak. The lower side 

 of this primitive streak is continuous with the mesoblast (mes), 

 while the entoderm (en) is here entirely distinct from the mesoderm. 

 It is evident that the mesoderm posterior to the blastopore is pro- 

 liferated from the lower side of the ectoblast and not from the upper 

 side of the entoblast, as is the case anterior to the blastopore. The 

 primitive groove gradually becomes more and more shallow, as it is 

 followed toward the posterior, until it is no longer discernible; 

 back of this point the primitive streak may be traced for a consider- 

 able distance, becoming thinner and thinner until it too disappears, 

 and there remains only the slightly thickened ectoblast underlaid by 

 the thin and irregular layers of mesoblast and entoblast. The prim- 

 itive streak may be traced for a distance equal to about one-third 

 the distance between the head-fold and the blastopore. 



Stack IV 

 Figures jn-yh (Plates VI, VII) 



No surface view of this stage was seen by the writer, and hence is 

 not figured. The figures were drawn from one of the series of sec- 

 tions obtained through the courtesy of Prof. S. F. Clarke. This 

 series was marked "3 Urwirbeln," so that the embryo was appar- 

 ently slightly younger than the youngest stage obtained by myself 

 and represented in figures 8 and 8a. 



Figure ja represents a section that passed through the head-fold 

 of the amnion (a) just in front of the head-fold of the embryo; the 

 amniotic cavity here appears as a large empty space. 



Figure jb is several sections posterior to the preceding; it passes 

 through the head-fold of the embryo, but is just back of the head- 

 fold of the amnion. The deep depression of the ectoderm (ec) and 

 entoderm (en) caused by the head-fold is plainly seen. In this de- 

 pression lie the ends of the medullary folds, distinct from each other 

 both dorsally and ventrally. Each medullary fold is made up of two 

 parts — a medial, more dense nervous layer (ill), and a distal, less 

 dense epidermal layer (ep). The section corresponding to this one 

 will be more fully described in connection with the following stage. 



Figure yc is some distance posterior to the preceding, though the 

 exact distance could not be determined because of a break in the 

 series at this point. The section passes through the posterior limit 

 of the head-fold. The medullary groove (mg) is very deep and 

 comparatively wide; around its sides the germ layers are so closely 

 associated that they may scarcely be distinguished. Ventral to the 

 medullary groove the foregut (fg) is seen as a crescentic slit. 



