8UBKINGD0M METAz'OA. 51 



which divide longitudinally in the usual manner, one half the 

 chromosomes of each nucleus passing towards one of the cen- 

 trosomes. The ovUm then divides into two cells and the 

 compound nucleus of each passes into the resting stage, the 

 chromosomes now uniting to form a single chromatic net- 

 work. 



It will be seen from this that the conjugation or fertiliza- 

 tion process consists of the union of two distinct nuclei, whose 

 complete fusion does not necessarily occur until after the first 

 division or segmentation of the ovum. 



A conjugation of centrosomes to form those of the first 

 segmentation-spindle also occurs. A centrosome accompanies 

 each of the conjugating nuclei (Kg. 23, A), and before the 

 formation of the spindle each divides into two (Fig. 23, B, oc 

 and sc), which conjugate in pairs (Fig. 23, O and D), forming 

 the centrosomes of the spindle, each of which thus contains 

 elements of both the original centrosomes. 



Furthermore, in some cases at least, it is possible to dis- 

 tinguish the naclear elements derived from the male and 

 female pronuclei respectively in stages later than the first 

 segmentation, owing to a slightly difi^ereut behavior to certain 

 staining reagents which characterizes them. The pronuclei 

 undergo a morphological fusion during the first cleavage of 

 the ovum, but a physiological differentiation persists. 



Segmentation and Early Development of the Ovum. — The 

 development of the ovum into the embryo consists in its divi- 

 sion into a number of cells, which gradually undergo a phy- 

 siological and morphological differentiation resulting in the 

 formation of tissues, organs, etc. These divisions constitute 

 the segmentation of the ovum. 



The first division has already been described ; it bears a 

 definite relation to the formation of the polar globules, the 

 plane of the division passing through the point at which they 

 were separated from the ovum. Considering this point to 

 represent one pole of the ovum, the first division is meridio- 

 nal, and the second division likewise, though its plane is at 

 right angles to that of the first division (Fig. 24, A). The 

 third division is, on the other hand, equatorial, its plane cutting 

 the planes of previous divisions at right angles (Fig. 24, B). 



