EMBRYOLOGY 401 



along the middle of it while its* raised edges are known as the 

 primitive folds. Its sides beneath the ectoderm are composed of 

 actively proliferating cells which give off their products into the 

 space between the ecto- and entoderm and so lay down the primitive 

 mesoderm cells. The front end of the primitive streak thickens 

 and swells out slightly to form the primitive knot or Henson's knot, 

 and just behind it the primitive groove terminates in a slight depres- 

 sion the primitive pit. The mesoderm is given off very rapidly from 

 the sides of the streak, passing out sideways, and it also aggregates 

 in the middle line beneath the streak and there comes in contact 

 with the underlying entoderm. This downward growth often makes 

 it appear in section as if this portion of the mesoderm were derived 

 from the entoderm. We have now reached a stage, then, in which 

 the three primary germ layers have been definitely established and 

 they are confined to the blastodermic area, being, as it were, unable 

 to stretch round and encompass the yolk which occupies the position 

 of the floor of the enteron. 



However, this triploblastic condition has not yet brought the 

 embryo up to the same stage of development as in Amphioxus and 

 Rana, so that we may trace it a little further and, as up to the present 

 there has been no indication of the appearance of a definite embryo, 

 the immediately succeeding stages are often referred to as the period 

 of formation or differentiation of the embryo. 



The mesoderm, as we have seen, is produced by multiplication 

 of the cells of the primitive streak ; indeed the streak itself is not 

 really a definite structure, but merely the result of an active prolifera- 

 tion and consequent aggregation of cells along this line. This meso- 

 derm spreads out laterally in the form of two wing-like plates which, 

 as they pass outwards, extend forward at their tips, and also finally 

 these anterior lateral extensions turn inwards and meet in the middle, 

 line. Their front edges, therefore, run first outwards, then forwards, 

 and finally inwards to meet well in front of the primitive knot, thus 

 there is left an area between their union and the end of the primitive 

 streak which is free of mesoderm and this region is termed the pro- 

 amnion. By this time not only has the lateral plate of the mesoderm 

 split into somatic and visceral layers, but in the latter have appeared 

 a number of compact masses of cells that give this region of the 

 blastoderm a characteristic mottled appearance. It is distinguished 

 from the remaining parts of the blastoderm as the area vasculosa, 

 since these masses, appearing as dark patches when the structure 

 is viewed as a whole, are destined to give rise to the first blood- 

 vessels and the blood itself. At a later stage the " blood islands," 

 as they are first termed, extend and anastomose forming an inter- 

 lacing series of blood-vessels over the whole of the area vasculosa. 



2 D 



