772 HANDBOOK OF PHYSIOLOGY'. 



tains a highly refractive body. Some large spheres contain a number of 

 spherules. Some of these are vacuolated. The white yolk not only en- 

 velopes the yellow yolk in a thin layer, and merges with the central 

 flask-shaped mass, already mentioned, but also is found in the yellow 

 yolk, forming with it alternate layers. 



Except that the central shining opacity of the pellucid area has dis- 

 appeared, that the size of the area has increased, and that the opaque 



Fig. 463, Transverse section through embryo chick (26 hours), a, epiblast; 6, mesoblast; 

 c, hypoblast ; d, central portion of mesoblast, which is here fused with epiblast ; e, primitive 

 groove ; /, dorsal ridge. (Klein.) 



area has also increased, no other change can be remarked up to the for- 

 mation of the two complete layers. There is, however, a slight ill- 

 defined opacity at the posterior part of the area pellucida, known as the 

 embryonic shield. This opacity is probably due to the intermediate cells 

 already mentioned as existing between the epiblast and hypoblast. 



In the posterior part of the area pellucida now appears an opaque 

 streak which extends about a third of the diameter of the area toward 

 the middle line. This is the Primitive streak. It is found on trans- 

 verse section of the blastoderm in this neighborhood to be due to a pro- 

 liferation downward of cells two or more deep from the epiblast. The 

 area pellucida now becomes oval. As the primitive streak becomes more 



m 10 



Fig, 464. Diagram of transverse section through an embryo before the closing-in of the 

 medullary groove, m, cells of epiblast lining the medullary groove which will form the spinal 

 cord; h. epiblast; d : hypoblast; eft, notochord; w, prptovertebra ; sp, mesoblast; u\ edge of 

 lamina dorsalis, folding over medullary groove. (Kolliker.) 



defined the area pellucida changes its oval for a pear shape, but the 

 streak increases in size faster than the area, and so after a time is about 

 two-thirds of its length. In the primitive streak a groove, the primi- 

 tive groove, runs along its axis. From the primitive streak the cells 

 from the under surface of the epiblast now extend as lateral wings to the 

 edge of the pellucid area; they are not joined with the hypoblast. The 



