178 STUDY OF YOUNG CHICK EMBRYOS. 



toward the median line of the embryo, one can distinguish two darker bands which 

 represent the beginning of the formation of the blood-vessels, growing in from the 

 extra-embryonic region to meet in the median line of the embryo and participate 

 in the formation of the heart. These bands are the anlages of the omphalo-mesa- 

 riac veins. Behind the fovea appear eight pairs of rather opaque blocks of tissue, 

 symmetrically placed right and left. These are the primitive segments and are 

 formed exclusively by the mesoderm. The first pair of blocks lie a short distance 

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

 sinus (compare below). When new segments are added they are about the same 

 size as those previously formed, and always arise at the caudal end of the series. 

 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 

 principal axial structure is the anlage of the central nervous system, the so-called 

 medullary groove, already partly converted into a medullary canal; for at this stage 

 it is closed from the anterior limit of the head to a variable point of the segmented 

 region of the embryo. For a general account of the origin of the medullary groove 

 from the ectoderm see page 67. Chicks with eight segments vary extremely as to 

 the extent of the closure of the groove. The line of .closure can be readily seen. 

 It is somewhat wavy and irregular in its course, and the closure itself is some- 

 what 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 the closure is always incomplete, there, being an 

 opening there which persists for some time and is known as the anterior neuropore. 

 Above the primitive segments, where it is not closed, the medullary 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 rhamboidal sinus. 

 The sinus marks the caudal limit of the nervous system and extends so as to em- 

 brace the cephalic end of the primitive streak. The region of the primitive streak 

 appears quite dark by transmitted light, owing to the accumulation of cells which 

 belong chiefly to the mesoderm. This dark appearance extends forward and 

 merges into a dark band on either side, which runs up to the row of segments. 

 The dark band is the segmental zone, out of which new segments are differentiated. 

 On the surface of the primitive streak is a longitudinal furrow, the primitive groove, 

 which begins just within the rhomboidal sinus and extends backward, often bend- 

 ing to one side or the other, usually to the left. The groove is shallow in front, 

 deeper behind, and ends quite abruptly. More careful examination of the area 

 opaca shows that it already possesses a well-defined area vasculosa, the peripheral 

 boundary of which is more or less definitely marked. In the fresh specimen only 

 traces of the formation of the blood-vessels and blood-islands can be made out. 



Examination of the Specimen after Hardening. The specimen, after it has been 

 hardened, should be examined under the microscope in water or alcohol; and, 

 again, after it has been stained it should be cleared in oil and further' examined. 



