CH. LIX.] 



SEGMENTATION 



831 



Segmentation. 



After fertilisation is completed, the ovum divides into two parts ; 

 each of these again divides, and so on till a mulberry-shaped mass 

 the morula is formed. It consists of a large number of small cells, 

 and it is enclosed together with the polar 

 bodies, in the zona pellucida. The polar 

 bodies soon disappear ; indeed in many cases 

 they have vanished long before the morula 

 is completed. A cavity soon appears in the 

 morula, which thus becomes converted into 

 a blastula or blastocyst. The cells which 

 form the peripheral wall of the blastula 

 assume a more or less cubical form, whilst 

 those which lie in the interior and form the 

 inner cell mass are irregular in outline, and 

 they are grouped together at one pole of the 

 blastula. At this period the blastula is 

 unilaininar, except at the region where the 

 inner cell mass is situated; but soon the 

 cells of the inner mass extend round the 

 cavity and the wall of the cyst becomes 

 bilaminar. In amphioxus and in many in- 

 vertebrates the blastula is at first entirely 

 unilaminar, no inner cell mass being present. 

 In these cases the inner layer is formed by 

 the invagination of a part of the wall of the 

 vesicle, and the opening at which the in- 

 vagination occurs is known as the blastopore 

 or primitive mouth. 



If the surface of a bilaminar mammalian 

 blastoderm is examined, an area is found 

 which is darker or more opaque than the 

 rest; this is the area where the embryo 

 will be formed, and it is known as the 



germinal or embryonic area (fig. 624). It corresponds with the 

 region where the inner mass is adherent to the outer layer, and 

 in it the epiblast cells are of cubical or columnar form, whilst over 

 the remainder of the wall of the vesicle they have been transformed 

 into flattened plates (fig. 625). At first the germinal area is circular, 

 then it becomes ovoid, and ultimately pear-shaped, the narrow part 

 of the pear-shaped area indicating the region of the posterior end of 

 the body of the future embryo (fig. 626). A linear streak the 

 primitive streak quickly appears in the narrow part of the area, 

 and after a time, a groove the primitive groove appears on its 



FIG. 623. Diagrams 



of th 



various stages of cleavage of 

 the ovum. (Dal ton.) 



