2 ELEMENTS OF HISTOLOGY. [chap. i. 



fact, its size is in proportion to its state of develop- 

 ment. 



2. Fertilisation causes marked changes in the 

 contractions of the protoplasm of the ovum ; these 



e 



Fig. 2. From a Section through the Blastoderm of Chick, uniucubated. 



a. Cells forming the ectoderm ; 6, cells forming the endoderm ; c, large 



formative cells ; /, segmentation cavity. (Handbook.) 



lead to cleavage or division of its body into two parts, 

 the germinal vesicle having previously split up into 

 two bodies or nuclei ; BO that we now find the ovum 



Fig. SA. From a Section through the Eudiment of the Embryo Chick. 



e, Primitive groove ; /, doreal laminae of epiblast ; d, mesoblast. The 



thin layer of spindle-shaped cells is the hypoblast. (Handbook.) 



has originated two new elements, each of which con- 

 sists of protoplasm, of the same substance as that of 

 the original ovum, and each contains one nucleus or 

 kernel. The investment of the ovum takes no part in 



