296 



STUDY OF YOUNG CHICK EMBRYOS. 



,"pro.ain 



in the median line of the embryo and participate in the formation of the heart on 

 the under side of the head. In order to see the fovea clearly, the focus of the 

 microscope must be lowered. The medullary groove is partly converted into the 

 medullary canal, for at this stage it is closed from the anterior limit of the head 

 to a variable point in the segmented region of the embryo. Specimens, however, 

 vary extremely as to the extent of the closure. The line of closure can be 

 readily seen. It is somewhat wavy and irregular in its course, and the closure 

 itself is somewhat irregular, so that we may find one or several points where the 



closure is not yet completed, although it 

 is complete behind and in front of these 

 points. At the anterior extremity of the 

 head closure is always incomplete, there 

 being an opening there which persists for 

 some time and is known as the anterior 

 neuropore. Above the primitive seg- 

 ments, where it is not closed, the medul- 

 lary groove has its edges close together. 

 But a short distance behind the last 

 segment the groove widens abruptly and 

 fades out gradually. This widening is 

 termed the rhomhoidal sinus. By proper 

 adjustment of the focus the notochord 

 may be distinguished underneath this 

 sinus. Just at the posterior limit of the 

 sinus the primitive groove, pr, begins and 

 extends backward, often bending to one 

 side or the other, usually to the left. The 

 groove is shallow in front, deeper behind, 

 and ends quite abruptly. The primitive 

 segments appear as square darkish blocks 

 of tissue symmetrically placed on either 

 side of the medullary canal. The first 

 pair of blocks lies a short distance behind 

 the fovea and the last pair a short distance in front of the rhomboidal sinus. 

 When new segments are added, they are about the same size as those previously 

 formed. The growth of the embryo in length during these stages depends 

 rather upon the multiplication of the segments than upon the growth of the 

 single segments. The region about the primitive streak is quite dark, owing 

 to the accumulation of cells, which belong chiefly to the mesoderm. This dark 



Fig. 167. — Chicken Embryo, after Twenty- 

 seven Hours' Incubation, with Eight 

 , Primitive Segments. 

 fov, Fovea cardiaca. pro. am, Pro-amnion. a.c.v, 

 Amnio-cardiac vesicle, st, Sinus terminalis. 

 pr. Primitive groove. Ao, Area opaca. A.p, 

 Area pellucida. — [After Duval.) 



