THE EMBRYONIC AREA. 23 



it takes part, as will be seen later, in the formation of the umbilical cord, which 

 connects the foetus with the placenta (p. 54). 



The Differentiation of the Embryonic Area. As the embryonic area is the 

 area of contact between the ecto-mesodermal and the entodermal vesicles it is, at 

 first, circular in outline. As growth continues the area becomes oval, and a linear 

 streak, the primitive streak, appears in that part of the oval which becomes the 

 posterior part of the area (Fig. 31). 



At the same time the position of the mesoderrnal elements of the wall of the 

 ecto-mesodermal vesicle is revealed, for the primitive streak is a thickened ridge of 

 cells which grows from the ecto-mesoderm and projects against the entoderm in the 

 posterior part of the embryonic area, pushing aside the primitive mesoderm which 

 intervened between the adjacent parts of the walls of the ecto-mesodermal and the 

 entodermal vesicles. The deeper cells of the ridge, those next the entoderm, are 

 the mesoderrnal elements of the primitive ecto-mesoderm, and, by proliferation, 

 they form the larger part, if not the whole, of the embryonic mesoderm and also an 

 organ, called the notochord. The mesoderm produced from the primitive streak 

 may be termed the secondary mesoderm. 



Immediately after the formation of the primitive streak a groove, the neural 

 groove, appears in the anterior 



part of the embryonic area. Embryonic area. 



It is formed by the longi- t plasmc " 



tudinal folding of a thickened Ch ] ceiiuiar 



Mesoderm lining-, 



plate of ectoderm, the neural I of trophobiast va 

 plate, which is the rudiment Mesoderrn of amnion; Jf 

 of the whole of the central 

 and peripheral portions of the Neural fold 

 nervous system, except per- 

 haps the olfactory nerves, and Ectoderm of ammon 

 the end organs of the sensory Neurenteric canal 

 nerves. From it also are de- 

 rived the cells of the primitive primitive streak 

 sheaths of the nerve -fibres 



and the chr omaffin cells of the M ?a?anic) SSr 

 supra-renal glands and other 



chromamn bodies. A 



rpi -i , i 11 f fv, FIG. 31. SCHEMA OF DORSAL SURFACE OF EMBRYONIC AREA OF 



ZYGOTE AFTER THE REMOVAL OF PART OF THE CHORION AND 

 neural groove are called the PART OF THE AMNION. 



neural folds. 



Almost from the first the anterior ends of the neural folds are united together 

 a short distance posterior to the anterior end of the embryonic area. Their 

 posterior ends, which remain separate for a time, embrace the anterior part of the 

 primitive streak. In the meantime, however, a groove, the primitive groove, has 

 appeared on the surface of the primitive streak. The anterior end of the primitive 

 groove deepens, until it forms a perforation which passes, through the anterior end 

 of the streak and the subjacent entoderm, into the cavity of the entodermal vesicle. 

 As this perforation passes from the floor of the posterior part of the neural groove 

 into that part of the entodermal vesicle which afterwards becomes the primitive 

 enteron or alimentary canal, it is called the neurenteric canal. The neurenteric 

 canal is but a transitory passage, and it disappears in man and other mammals 

 before the neural groove is converted into a closed neural tube. 



After the appearance of the primitive groove and the neurenteric canal the 

 posterior ends of the neural folds converge, across the anterior part of the primitive 

 streak and groove, and fuse together posterior to the neurenteric canal. The 

 primitive streak is thus divided into two portions. (1) An anterior portion, which 

 lies at first in the floor of the neural groove, and, later, in the floor or ventral wall 

 of the posterior end of the spinal medulla; and (2) a posterior portion, which 

 remains on the surface and takes part in the formation of the median portion of 

 the posterior end of the body, forming the perineum, and the median part of the 

 ventral wall of the body, from the perineum to the umbilicus. It is through the 



