16 GENERAL EMBRYOLOGY. 
half-centrosomes are formed, two male and two female. From the four half-centrosomes two new 
centrosomes are formed by the union of half a male centrosome with half a female centrosome. 
Each of the two centrosomes, therefore, which accompany the first segmentation nucleus contains 
both male and female elements 
A B 
MP 
SFP 
Fria. 8.—FERTILISATION OF THE OvuM (Diagrammatic). 
A, The entrance of the spermatozoon and the formation of the cone of attraction; B, The appearance of the 
centrosome ; C, The approachment of the male and female pronuclei ; D, The first segmentation nucleus, 
C. Centrosome. FP. Female pronucleus, P. Polar body. 
CA. Cone of attraction. MP. Male pronucleus. SN. Segmentation nucleus. 
Segmentation.—Segmentation is the division of the fertilised ovum (oosperm) 
into a number of cells. These cells are afterwards arranged in layers—the germinal 
layers or layers of the blastoderm; ultimately they are differentiated into the tissue 
elements of the body. 
Fic. 9.—SEGMENTATION OF THE FERTILISED OVUM IN THE RABBIT. 
Formation of blastomeres and morula (Diagrammatic). 
A, Division into two segments ; B, Division into four segments ; C, Morula. 
P, Polar bodies, 
The phenomena of segmentation have not been observed in the human ovum, 
and it is to be understood that the following description is based chiefly upon the 
conditions met with in rodents, more especially in the rabbit, an animal well 
adapted for the study of these phenomena. 
