GENERAL EMBRYOLOGY 1615 



and in line with the primitive streak, which constitutes the head- 

 process of the streak. 



Significance of the Head- Process and Hensen's Node. — The ectodermic 

 cells, which lie over the head-process, give rise to the medullary plate, neural 

 or medullary groove, medullar^' folds, and neural or medullary canal, the 

 last named being the rudiment of the entire cerebro-spinal nervous system. 

 The posterior end of the neural groove embraces the anterior end of the 

 primitive streak and primitive groove. In this situation, where Hensen's 

 node is located, a somewhat wide communication exists at an early period 

 between the posterior end of the neural canal and the primitive intestinjil 

 cavity. This communication, which is temporary, is known as the neoren- 

 teric canal. It passes through Hensen's node, and it corresponds to the 

 blastopore of Amphioxus. 



Along the under aspect of the ectodermic cells, which give rise to the neural 

 canal, the cells of the subjacent entoderm become thickened and differentiated 

 to form the notochordal plate. This plate gives rise to the cylinder of ceUs. 

 called the notochord, which is the axis round which the vertebral column 

 and its hgaments are developed. 



The posterior part of the primitive streak gives rise to the cloacal 

 membrane, which is bilaminar, one layer consisting of ectoderm, 

 and the other of entoderm. It forms the posterior boundary of the 

 primitive alimentary tube, and is subsequently concerned in the 

 formation of the anus. 



Formation of the Mesoderm. — On either side of the primitive 

 groove the cells of the primitive streak become proliferous. Cell- 

 proliferation also occurs on either side of the head-process. The 

 new cells, thus formed, gradually extend, on either side, into the 

 blastoderm, and insinuate themselves hetween its two layers, which 

 consist of ectoderm and entoderm respectively. In this manner a 

 third or middle layer is formed for the blastoderm, and this laver, 

 which is of secondary formation, represents the primitive mesoderm. 

 It is chiefly derived from the primitive streak and its head-process, 

 which consist of embr^-onic ectoderm; but it is reinforced by cells 

 from the subjacent entoderm. The mesoderm is thus a secondary 

 formation, whilst the ectoderm and entoderm are of primary origin. 



Inasmuch as the embryonic ectoderm and entoderm are derived 

 from the inner cell-mass, it follows that the inner cell-mass is con- 

 cerned in the formation of all three germinal layers of the embryo. 



The blastodermic vesicle has now a trilaminar blastoderm, the 

 layers, from without inwards, being the ectoderm, the mesoderm, 

 and the entoderm. 



Summary of the Formation of the Germinal Layers. 



The germinal layers are three in number — namely, ectoderm, entoderm, 

 and mesoderm. The ectoderm and entoderm are of primary origin, whilst 

 the mesoderm is of secondary origin. The ectoderm is developed from the 

 outer layer of blastomeres or segmentation-ceUs, which result from the 

 segmentation of the fertilized ovum, and the entoderm is developed from the 

 inner cell-mass. The mesoderm is developed chiefly from the primitive 

 streak on the embryonic area of the blastodermic vesicle, the cells of which 

 streak consist of ectoderm, but it also derives cells from the entoderm which 

 lies close beneath the primitive streak. The mesoderm is also, in part, derived 

 from the head-process of the primitive streak. 



