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Monitoring Stem Cell MsEmcm 
Figure 9D: Photo micrograph of Primitive Streak 
[wv7w.viseiiibryo.com by Mouseworks, Inc.] 
Vn. NEURULATION \ 
Neumlation is the series of developmental events that result in 
the beginnings of the central nervous system (Figure 10). From the 
cranial end of the primitive streak, a long stiff structure develops in j 
the mesoderm, elongating still further in the cranial direction. This I 
becomes the notochord, wrhich marks the head/tail axis of the | 
embryo. Later, the vertebral column develops around it. But at this J 
time, the notochord and its adjacent tissue exert influence called j 
primary induction on the ectoderm lying over them, such that the 
ectoderm thickens and becomes the neural plate. I 
The neural plate then actually pushes up to form folds (called the 
neural folds) along each side of the tissue over the notochord. The 
neural folds then meet and fuse to enclose the neural tube, beginning 
at the middle of the (future) tube, like a zipper closing from the 
middle toward each end. This process is completed by the end of the 
third week. Some cells along the crests of the folds migrate through 
the embryo. They are called neural crest cells, and they give rise to a 
variety of nerve cells including dorsal root (spinal) and autonomic 
nervous system ganglia, and some other nervous system and 
endocrine structures. 
PRE-PUBLICATION VERSION 
