OH. LVIII.] 



SEGMENTATION. 



807 



formation of the second layer within the first differs in different 

 animals. In amphioxus and many of the invertebrates, it is 

 formed by a process of invagination ; that is, a portion of the 

 surface layer is gradually pushed in until it completes a second 

 layer within the first ; the orifice of invagination is called the 

 blastopore, and corresponds to a primitive mouth opening into a 



primitive alimentary cavity sur- 

 rounded by the inner layer of cells. 

 In mammals, the cells which are 

 going to form the second layer take 

 up a central position from the very 

 start, and the outer cells by multi- 



Fig. 6/9. Impregnated egg, with 

 commeucement of formation of 

 embryo ; showing the area ger- 

 minativa or embryonic spot, the 

 area pellucida, and the primitive 

 groove and streak. (Dal ton.) 



plying more quickly than the inner 

 ones grow round and enclose them. 

 This is the gastrula stage or the 

 stage of the bilaminar blastoderm. 

 Then a third layer is formed be- 

 tween the other two, and thus we 

 arrive at the stage of the trilaminar 



blastoderm. The three layers are called from without inwards 

 the epiblast, mesoblast, and hypoblast, or the ectoderm, mesoderm, 

 and tndoderm. 



We must next study the way in which the mesoblast is formed 

 between the other two layers. When the outer surface of the ovum 

 is viewed from above, a streak or shadow is visible ; this occurs 

 first at its posterior end, and it gradually extends towards the 



Fig. 618. Diagrams of the varioux 

 taxes of cleavage of the ovum. 

 (Dal ton.) 



